How can you change the Xbox Gamertag via the Xbox App I want to use Google Play credit
Didn't help at all
If you'd like to change the gamertag that was automatically created for you when you signed up (i.e., you didn't choose it yourself), you can change it once for free in the Xbox Console Companion app on Windows 10.
Note: If you've changed your gamertag before, go to the Select New Gamertag page to change it again. For steps, see the "Changing the Gamertag Using a Web Browser" section on this page.
Sign in to the Xbox Console Companion app on Windows 10 using the email address and password of the Microsoft account associated with your gamertag.
Select your gamerpic in the top left corner of the app's home page.
Under your profile name, select Customize> Change Gamertag.
Enter your preferred gamertag and select Check Availability.
Once you find an available gamertag, select the Claim it option.
After confirming the new gamertag, the change will be applied to Xbox. If there are additional costs, the payment option associated with your Microsoft account will be charged. You don't need to notify your friends of the changed gamertag. The friend lists are updated automatically.
So it says on the support page
Win 10 has nothing to do with Android 9 mobile phone