To manage Autopay, you must be logged in as a registered user. Logging in as a guest will not allow you to make changes to Autopay.
After you make a payment, the system will ask if you would like to store your credit/debit card and enable Autopay. Autopay will debit your card on file based on the payment schedule created at the time of the vehicle purchase.
You can find the Autopay settings under account settings.