Instructions
Log in to your Xbox LIVE account using your current gamertag.
Scroll to the "My Xbox" menu.
Scroll to your current gamertag and select it.
Choose "Change Gamertag" from the menu.
Enter your new gamertag. If you do not have a new name yet, you can choose "Get Suggestions" for possible ideas.
Choose "Yes, use this gamertag" once you have created your new gamertag for your Xbox LIVE account. Microsoft will deduct your "Microsoft Points" automatically. If you do not have enough points, select "Add Microsoft Points" from the menu and Microsoft will charge a fee to the credit card that you use to pay for your Xbox Live subscription.