To update your credit card, please log into your Billing Area. If this is your first time logging in you can click the Forgot your password? button. From there you can enter the email address that you used to make your purchase. An email will be sent to you to update your password.
Once logged, click on Update My Billing Info, then click the blue button to Add a New Card. You may also edit or delete any existing cards from this screen as well.
Here is a quick video walk-through of these steps.