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For Christ sake GOOGLE BEFORE ASKING.

http://support.apple.com/kb/HE40

If that does not work contact either iTunes Store Support, iOS App Store Support or Mac App Store Support and ask.

If they can help you, then they will. If not you might be stuck with that ID.
 
you can also go to the apple store, click account > change account information. login and change it. basically the same as stated above.
 
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