It depends on what you want to change it to. If you want to change it to a pre-existing but unused ID, you simply can't. I had this very problem and even though the ID I wanted (
[email protected]) was not in use, it had once upon a time been allocated, and .Mac retains these addresses permanently, in case the original owner of the ID decides to rejoin.
If you want to change it to a new, unused one, you can either (a) follow the support links on the .Mac page, get a live chat going with a support person and fix it that way, or (b) take out a new .Mac ID which has the ID you want.
Those are the options I am aware of!