Have a look at the Accepted answer over here:
[https://learn.microsoft.com/en-us/answers/questions/2338169/need-help-changing-my-alias-(@xboxlivefamily-com-a](https://learn.microsoft.com/en-us/answers/questions/2338169/need-help-changing-my-alias-(@xboxlivefamily-com-a)
Or try Xbox support: