Updating connection string in app.config permanenlty from C#

Question

I am facing problem in writing/ updating connection string in app.config file using c# codebehind . I applied different solutions but no solution could save connection string permanently in app.config. All solutions save connection string temporarily in app.config.exe which lost data when application exits.
 
Any help will be highly appreciated.
 

Regards,
 
Shahzad