Hello,
I've already encounter this kind of problem.
First the file is probably in "reading only" (don't know if it is correct, in france it is lecture seule) which means you can't do modification on the file. So you must deactivate this property of the file (right click, properties, etc.)
Sometimes it's not enough, so i make a copy of the file in another directory, modify this one and replace the first once i have finished. (something i think linked to the properties of directories, but i'm not sure)
Try it, and tell me if it has worked, or not.