change the Connection string in Linq

Hi

I used the "linq to sql" to connect to the server and retrieve some  data ,

but i have one problem that , when i used this program in client System in the network , and the database is in server , i can't connect it.

 I know that i must change the "conncetin string" to do this , and i know the" linq to sql" class was saved the  "connection string" ,that use to connect to the server into (App.confiq  & Properties.Setting)

I dont know which one of them , i must be change (App.config or Properties.Setting )

I forgat that say , I Useed  the C# to write this App.