Follow these steps to change your gamertag. Note that the first time you change your gamertag, there’s no charge if done within 30 days of creating your account. However, there will be a small charge for all subsequent changes.
Change your gamertag
Select your profile on the Home screen.
Select My profile.
Select Set gamertag.
Choose a gamertag.
Or, you can select Make my own to create your own gamertag.
Confirm the gamertag change by clicking Sounds good.