Drag the file out of the Finder window and place it somewhere as a backup. Then, do the same thing again, only drag the file to your desktop. You now how two backups of the file, but you'll be changing one in a second. Okay, open the file on the desktop in TextEdit. Do the necessary changes and save. Now, drag this file back into the original Finder window with the original file. You'll be asked if you want to replace the existing file, say yes. You'll have to authenticate with your password. You're done.