For us to pay the balance of your device, you can log in to https://www.t-mobile.com using your T-mobile online account.
Go to Account tab 🡆 go to Lines and Devices 🡆 click your device/line 🡆 Under Equipment Financing click the price and you will be redirected to a page where you can pay the remaining balance of your device by clicking Make a payment. Just add a payment method and submit.
If you are using the T-mobile app, just go to bill 🡆 Equipment installation plans and click payment.