For those already with a custom Gamertag, changing it requires a small fee to be paid. At $9.99, or an equivalent value in your local currency, you'll be granted one Gamertag change. How to change...How do you change your Gamertag for free?
In a web browser, go to Change gamertag at account.xbox.com. Sign in using the Microsoft account email address and password associated with your gamertag. Enter your new gamertag and click or tap Check Availability. Once you find an available gamertag, click Claim it! Note You can change your gamertag once for free.