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these instructions should work. If they don't please tell me exactly what error you get. I can even log in remotely to do it with you if you want to pay a little bit extra.
You can easily add or edit an Xbox Live payment option on Xbox.com. Here’s how:
Note If you don't know your Microsoft account or password, please see Manage your Microsoft account.
If you have more than one payment option, click Apply to Membership next to the payment option that you want to use for your me