WPF, Entity Framework, Conection String

Hello guys,

I have a code:

EntityConnectionStringBuilder efb = new EntityConnectionStringBuilder(ConfigurationManager.ConnectionStrings["AutoLotEntities"].ConnectionString);
SqlConnectionStringBuilder newBuilder = new SqlConnectionStringBuilder(efb.ProviderConnectionString);
newbuilder.UserID = "new_user";
sqlcsBuilder.Password = "new_pass";

now how should I upgrade/save connection string in app.config?