messing with .cfg files

Greetings all. I need some help. I found a skin I'd like to add to Mike Stone's C-130 for my FS2004. In the read me it says that I must make alterations to the aircraft.cfg file.
About that change - a line says " add the following to the aircraft.cfg file sustituting the next numbers instead of x in the line [fltsim.x] where appropriate."
sustituting
I understand you can open this file up in notepad but how are you able to make the change and still have it saved as a .cfg file? The default is a text file without the option of .cfg. Is there a better program for editing?
Can anyone make better sense of these instructions for me?
Thanks
About that change - a line says " add the following to the aircraft.cfg file sustituting the next numbers instead of x in the line [fltsim.x] where appropriate."
sustituting

I understand you can open this file up in notepad but how are you able to make the change and still have it saved as a .cfg file? The default is a text file without the option of .cfg. Is there a better program for editing?
Can anyone make better sense of these instructions for me?

Thanks