You can change your credit card details under the My Cards tabs under My Account.
Add a new credit card -
To add a new card, click the green 'Add card' button on the top left of the page.
Then fill in the fields and click 'Save changes' once you're done.
Deleting a credit card -
To delete an existing credit card, you need to click the bin icon located on the right hand side of the page and click 'Yes' when it prompts.