I imagine a program like gedit would probably be a good for editing the .xml files. Are you trying to edit it on your phone or on your computer? If it's on your computer then you should just be able to:
That will pop up a root version of notepad that should let you edit it. I have no idea if editing it on your phone directly (if possible) will cause problems though or if that is why you're not able to edit the file.
$ sudo gedit /path/to/xml