XML : Little utility to add element in .config app file

I need to make a little utility that upgrade an xml .config c# application file outside that application.

I just need to add this element:

  <?xml version="1.0"?>    <configuration>      ...      ...      <startup useLegacyV2RuntimeActivationPolicy="true">      <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5.2"/>    </startup>  </configuration>    

Wich is the easy way?