You can change this by going to the Account tab in the left side menu of your members area, clicking on the 'Change Billing Info' option, entering the new credit card information in the fields provided and clicking the 'Update Billing Info' button at the bottom of the form.
If you are having a hard time updating your information, you may have to log out of your account completely and try again, or try using a different web browser.
Note: When you update the card on file a $1 verification charge will be billed to your card and then refunded. If you have a past-due balance, your card will be charged for the past-due amount when the card is updated.