@anjou_riya, my guess is that you're trying to edit something from inside the WP itself.
In order to edit all the necessary configuration files, you will need either ssh access through command-line tool or you can use an application (e.g. CyberDuck or FileZilla). Please read this guide how to do it.
In case you'll encounter issues with the domain, read my other post from here to find how to do it manually.
Best of luck!