Instructions
Power on your Xbox 360 console and sign in the gamertag account that you wish to change if you do not sign in automatically.
Navigate to the "My Xbox" blade from the main console menu and select your gamertag.
Press the control stick right to move to the "Account Management" tab. Note your current Microsoft Point balance.
Select "Add Microsoft Points" and purchase the appropriate amount of Microsoft Points. If you already have some Microsoft Points, you may only need to select the 400 Microsoft Points option. If not, select the 800 Microsoft Points option, which retails for $9.99.
Return to the gamertag page and select "Change Gamertag."
Select "Enter New Gamertag" and enter the new gamertag of your choice.
Select "Done" to continue and confirm that you wish to spend 800 Microsoft Points to redeem a new gamertag.