C# default connection string problem

Question

 I am having a problem with the default behavior of linq to sql when I added a new table to the app.dbml file to an existing C# 2010 app.  The application changed the connection string from appConnectionString (Settings) to OtherConnectionString (Settings).

I am trying to get the app.dbml file file to allow me to change it back to appConnectionString (Settings). However the designer is not allowing me to do that unless i create a new connection string.

Basically I am opening the  app.dbml file so I can see the designer. From here I click on properties and I see the connection string settings.

Thus can you tell me how I can get access to change the setting back to  appConnectionString (Settings)?