You can't write to the XML-file from Flash - only read the contents into Flash. If you want to modify the contents, then you'll need to do this server-side.
You'll find excellent tutorials at
http://www.flashkit.com and
http://www.ultrashock.com - just search for "xml". Remember that XML parsing in Flash 5 is quite slow unless you use Branden Hall's library. However, XML parsing in Flash MX has been drastically improved, so I would definately recommend that you upgrade, if you haven't done this already.
Cheers,
Morten