contact T-Mobile and change it.
Hey there! It is so nice of you to be taking care of your niece's line... and protecting her. You can find out a little more about number changes here: Change your phone number - but ultimately, @tidbits is correct - this is something we'll need to take care of for you over the phone. If she's on a postpaid plan under your name that should be an easy call, but if you're covering a Pay In Advance line for her, you may need to make sure you know her PIN first! Thanks for reaching out here, I hope everything's going well for you both.