Log into the Customer Portal and click on My Service (1). Then click on Payment Options (2).
Click Add Payment Method.
You can now enter your new credit card details. Click on Submit (1) to use the new card, or Cancel (2) if you decide you don't want to add a new card.
You'll be asked to confirm your changes. Click Yes, and you'll see a green banner notifying you that your payment method has been successfully updated. You'll also receive an email confirming the change.