To make life easier you can do one of two things:
1) copy the file to the desktop, open it there, make your mods, save and close it, then move it back where it came from, overwriting the old one. If you want, a good idea is tto make a backup first in case something goes wrong with the mods.
I suspect your AIC is in C:\Program Files or \Program Files(X86). Bad place to put stuff, as this problem arises. Same for modifying aircraft.cfg, panel.cfg and so on. Much better to place it in C:\, or even a folder on D:\ or E:\ or wherever, just NOT one of those two folders.
2) Make a shortcut to your Notepad or Notepad++ on the desktop, and go to it's Properties>Advanced and click the box for Run As Administrator. That way, it will always open, using that shortcut, as Admin, and you don't have to click the Run As Admin every time you open it. I do this with just about every program I use often.
IMHO, your best bet, though, is to never install any program you use often, or have to edit parts of, into the Program Files or Program Files(X86). It's fine for Windows "stuff" but not at all good for programs users may want to modify in any way. And if you open it from a shortcut, always always always make sure that Run As Admin is checked so it always opens as Admin. It really helps, and I do it out of habit any-more.
Hope that helps a little. I know, I tend to go on sometimes. Sorry if I over-explain sometimes...
Pat☺