You can change the payment information associated with your account at any time on your account page.
Click on the 'Billing' tab. You'll see the cc info or PayPal email address that is currently associated with the account. Click on the little pencil icon to the right of your cc info to EDIT.
Not all graphic cards will show the pencil and X icons on the Billing page. If you are unable to see it, hover the mouse in the general area of the icon and you should be able to click on the icons.
Clicking near the pencil icon will give you the ability to edit your credit card.