After you have signed into your account you can then view your account information.
First click on "Membership" in the top right of the screen.
This will then take you to your account tabs, like below.
Once you are here, click on the "Account Info".
From this page click on "Edit your credit card".
Then a box will appear to for you input the credit card info. Once the info is entered click "Apply"
Please note that updating your credit card is also the same way to update your billing address listed under the "Account Info" tab.