You can update your payment details through My Account.
Click on to the "profile" section of my account and you'll come across "my bank details" at the bottom. You'll have to sign in again to view or make changes to your payment details. Once updated, click "done" and your new payment details will be saved.
Please note: once you've confirmed your order, you won't be able to change your payment method. We'd advise double checking everything is correct before confirming.