A simple example of how to add and replace web.config settings in transforms

web.config.transform.xml

<configuration xmlns:xdt="http://schemas.microsoft.com/XML-Document-Transform">
 
  <appSettings>

    <!-- adds -->
    <add key="Key.To.Insert" value="The-Value-To-Insert" xdt:Transform="Insert"  />
  
    <!-- replaces-->
  
    <add key="Environment" value="Release" xdt:Transform="Replace" xdt:Locator="Match(key)" />
    

  </appSettings>
   
</configuration>