How to Programmatically Update an ODC File - https://blogs.msdn.microsoft.com/sharepointdev/2012/02/09/how-to-programmatically-update-an-odc-file-shubh-raj-singh/

UpdateODC.cs

/*
The following is a simple console application that shows how to change values inside an ODC file. This code can be used in a feature or a console application with all the previous parameter values provided from a configuration file. This code changes the highlighted values in the data connection XML shown above.
 */

namespace UpdateODCFile
{
    using System.Linq;
    using System.Text;
    using System.Xml;

    /// <summary> 
    /// Class to change the ODC file parameters 
    /// </summary>

    class Program
    {

        /// <summary> 
        /// Index from where tag begins 
        /// </summary>

        private static int sourceIndex = 0;

        /// <summary> 
        /// Index from where tag ends 
        /// </summary>

        private static int destinationIndex = 0;

        static void Main(string[] args)
        {
            // You can keep the source an destination file path same if you want to 
            // overwrite the existing file with new values.

            string odcFilePath = @"C: ReportsConnection.odc";
            string destinationFilePath = @"C: ReportsConnection.odc";

            // Provide values for following patameters that will be changed in the ODC file.

            string databaseName = "< your database name>";
            string serverName = "< your server name>";
            string SSOApplicationID = "< application ID >"; string provider = "< Provider >";
            string integratedSecurity = "< integrated security >";
            string persistSecurityInfo = "< persist security info>";
            string useProcedure = "< use procedure >";
            string autoTranslate = "< auto translate >";
            string packetSize = "< packet size >";
            string workSatationID = "< workstation ID >";
            string encryptionData = "< encryption data >";
            string tagWithCollation = "< tag with column collation >";

            // The following code reads the ODC file and retrieves the ODC connection XML data from it.

            string xmlConnection = GetConnectionString(odcFilePath);
            XmlDocument odcXmlConnection = new XmlDocument();
            odcXmlConnection.LoadXml(xmlConnection);
            XmlNamespaceManager nameManager = new XmlNamespaceManager(odcXmlConnection.NameTable);
            nameManager.AddNamespace("odc", odcXmlConnection.DocumentElement.NamespaceURI);
            XmlNodeList nodelistConnectionString = odcXmlConnection.SelectNodes("//odc:Connection/odc:ConnectionString", nameManager);

// The following code changes the connection properties mentioned above.

StringBuilder finalConnectionString = CreateNewConnectionString(databaseName, serverName, provider, integratedSecurity, persistSecurityInfo, useProcedure, autoTranslate, packetSize, workSatationID, encryptionData, tagWithCollation, nodelistConnectionString);
            nodelistConnectionString[0].InnerText = finalConnectionString.ToString();

            // The following code changes the SSOApplicationID property in the XML.

            XmlNodeList nodelistSSOApplicationID = odcXmlConnection.SelectNodes("//odc:Connection/odc:SSOApplicationID", nameManager); 
nodelistSSOApplicationID[0].InnerText = SSOApplicationID;

            // The following code saves the updated file on to a disk. When this is complete, the file can be published to SharePoint.

            SaveConnectionString(odcFilePath, destinationFilePath, odcXmlConnection.OuterXml);
        }

        // The following code finds the connection string tag in the ODC file and returns the full connection string.

        /// <summary> 
        /// Finds the connection string in the ODC file 
        /// </summary>

        /// <param name="filePath">path of ODC file</param> 
        /// <returns>office data connection xml</returns>

        public static string GetConnectionString(string filePath)
        {
            string xmlConnection = string.Empty;
            System.IO.StreamReader myFile = new System.IO.StreamReader(filePath);
            string myString = myFile.ReadToEnd();
            sourceIndex = myString.IndexOf("< odc:OfficeDataConnection");
            destinationIndex = myString.IndexOf("</ odc:OfficeDataConnection");
            xmlConnection = myString.Substring(sourceIndex, destinationIndex - sourceIndex + 27);
            myFile.Close();

            return xmlConnection;

        }

        // The following code saves the connection string tag to the ODC file and saves it to a disk.

        /// <summary> 
        /// save the changed ODC file 
        /// </summary>

        /// <param name="filePath">path at which you want to read the ODC file</param> 
        /// <param name="newFilePath">path at which you want to save</param> 
        /// <param name="connectionString">new data connection string to be changed</param>

        public static void SaveConnectionString(string filePath, string newFilePath, string connectionString)
        {
            string xmlConnection = string.Empty;
            System.IO.StreamReader myFile = new System.IO.StreamReader(filePath);
            string myString = myFile.ReadToEnd();
            myFile.Close();
            string lessString = myString.Remove(sourceIndex, destinationIndex - sourceIndex + 27);
            myString = lessString.Insert(sourceIndex, connectionString);
            System.IO.StreamWriter writer = new System.IO.StreamWriter(newFilePath);
            writer.Write(myString);
        }

        // The following code finds the properties in the ODC file and then changes them. If a property does not require any changes, it can be removed from this code. The new connection string will be returned to the Main function.

        /// <summary> 
        /// Function to replace the old values with new one. 
        /// </summary>

        /// <param name="databaseName">database Name</param> 
        /// <param name="serverName">server Name</param> 
        /// <param name="provider">provider</param> 
        /// <param name="integratedSecurity">integrated Security</param> 
        /// <param name="persistSecurityInfo">persist Security Info</param> 
        /// <param name="useProcedure">use Procedure</param> 
        /// <param name="autoTranslate">auto Translate</param> 
        /// <param name="packetSize">packet Size</param> 
        /// <param name="workSatationID">work Satation ID</param> 
        /// <param name="encryptionData">encryption Data</param> 
        /// <param name="tagWithCollation">tag With Collation</param> 
        /// <param name="nodelistConnectionString">nodelist Connection String</param> 
        /// <returns>final connection string</returns>

        private static StringBuilder CreateNewConnectionString(string databaseName, string serverName, string provider, string integratedSecurity, string persistSecurityInfo, string useProcedure, string autoTranslate, string packetSize, string workSatationID, string encryptionData, string tagWithCollation, XmlNodeList nodelistConnectionString)
        {
            string[] connectionStringArray = nodelistConnectionString[0].InnerText.Split(';');
            StringBuilder finalConnectionString = new StringBuilder();
            foreach (string connections in connectionStringArray.ToList())
            {
                string[] splitOnEqual = connections.Split('=');
                switch (splitOnEqual[0])
                {
                    case "Initial Catalog": splitOnEqual[1] = databaseName;
                        break;

                    case "Data Source": splitOnEqual[1] = serverName;
                        break;

                    case "Provider": splitOnEqual[1] = provider;
                        break;

                    case "Integrated Security": splitOnEqual[1] = integratedSecurity;
                        break;

                    case "Persist Security Info": splitOnEqual[1] = persistSecurityInfo;
                        break;

                    case "Use Procedure for Prepare": splitOnEqual[1] = useProcedure;
                    break;

                    case "Auto Translate": splitOnEqual[1] = autoTranslate;
                        break;

                    case "Packet Size": splitOnEqual[1] = packetSize;
                        break;

                    case "Workstation ID": splitOnEqual[1] = workSatationID;
                        break;

                    case "Use Encryption for Data": splitOnEqual[1] = encryptionData;
                    break;

                    case "Tag with column collation when possible": splitOnEqual[1] = tagWithCollation;
                        break;

                }

                finalConnectionString = finalConnectionString.Append(splitOnEqual[0]);
                finalConnectionString = finalConnectionString.Append("=");
                finalConnectionString = finalConnectionString.Append(splitOnEqual[1]);
                finalConnectionString = finalConnectionString.Append(";");

            }

            finalConnectionString = finalConnectionString.Remove((finalConnectionString.Length - 1), 1);
            return finalConnectionString;

        }
    }
}