To update the existing credit card, click on Settings Icon Account Settings option in the user menu

Next click the Billing and Usage option.

Now click on the edit icon for the Payment Details

Now fill in your information and hit save. If there are any errors, you will be informed for each field.  You can update individual details about the credit card without re-entering the sensitive account numbers.  But if you modify the credit card number or CVC you will need to re-enter them.