If you want to change the student loan being paid by Digit or update your biller's address you'll first need to unlink the current account.
Removing the current account
1) Tap into Settings to bring up your goal menu
2) Select Manage Payments
4) At the bottom of the next screen, tap Disconnect Account, and then confirm that you want to disconnect
After that account is removed, you will see a banner asking if you want to connect an account to your goal.