ModifyConnectionstring.cs

public void ChangeConnectionString(string key)
{
    var config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
    var connectionStringsSection = (ConnectionStringsSection)config.GetSection("connectionStrings");
    var value = connectionStringsSection.ConnectionStrings[key];
    connectionStringsSection.ConnectionStrings[key].ConnectionString = ConnectionString;
    config.Save(ConfigurationSaveMode.Modified, true);
    ConfigurationManager.RefreshSection("connectionStrings");
}