I changed my Xbox username so i unlinked my account and relinked it through the website but now on the mobile app It won’t change and it won’t let me link my new username profile.
Have you tried unlinking the account on the mobile app? To do this, you can navigate to More > Settings and unlink the profile there.
After that, you can navigate back to the My Profile page and attempt to relink your profile.
Hope this helps!