To change where Digit is sending payments, or if the remittance address of your loan servicer has changed, you'll first need to unlink the account that your goal is currently connected to.
Removing the current account
1) Tap into your student loan goal from the Digit app
2) Tap into Settings to bring up your goal menu
3) 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 should see a banner asking if you want to connect an account to your goal.