You can update your credit card information under "My Account" within your Dashboard.
To update or add a new credit card, first be sure you are logged in to your account.
Click "My Account" at the top of the page, which takes you to "My Dashboard."
On the left hand side of the page, click "My Payment Methods."
Now you should see your "Stored Payment Methods," which are the cards you currently have on file.
Go Click "Edit" under the BILL TO information. Choose to add your new card information as the default payment for your subscription.
Be sure to also update the billing address if necessary.