To update your payment method you need to remove the existing credit card from your account, under "My Account - Saved Cards - Delete"
Then add the new payment method.
(Your credit card is automatically saved as your default payment option, if you wish to remove it, you can do so through the "Saved Cards" option in your account. This option is not available for Autodeliver customers as per the Terms and Conditions of the Autodeliver Policy)