If you DID NOT use PayPal then you can update your payment method through the Guide Sites account portal. If you DID use your PayPal account to join a Guide Site then you have to update your payment information inside of PayPal.
To go to your "Guide Sites" account portal then click on this link and log-in. (NOTE: It's a good idea to bookmark the Guide Sites account portal.
After you log in click on the link that says "Update Credit Card Info".
You can then update your credit card information if you DID NOT use a PayPal account to join.