Updating Another Applications App.config Key Value Pair

I'm trying to modify an app.config file that is not part of my .net application.My key needs to be unique and appear under a static key (Server1 or Server2) as the other application reads these values line by line, associating the database(n) value with the server(n) value.I cannot see a method of using the standard app.config methods against an external .config file, so I'm using the Xml Document class.[code]But cannot find a method of retriving the Key to check if it exists and more importantly increment the value?