You can change your credit card details under My Account and the My Cards tab.
Add a new credit card -
To add a new card, click on the Subscriptions tab, click the Manage button and select Update renewal payment method
Select Add new card, enter your details and click Create:
Deleting a credit card -
To delete an existing credit card, please go to the My Cards tab and click the bin icon located on the right hand side of the page and click Yes when it prompts.