Hi Sundrop. First - make sure the file you're editing is not configured as Read-only. Right-click on the icon & select Properties. Uncheck the Read-only option if necessary.
I suggest associating CFG files with Notepad. "Associating File Extensions" here.
http://www.simviation.com/lair/windows.htmThis makes it so easy as a double-click on its icon will then open any CFG file in Notepad. After editing, exit Notepad saving changes when prompted. It's as simple as that.

PS. Please take a backup copy of any file before editing it in case of mistakes.