To update the card on file for future payments, you can update this in two ways.
If you have an invoice due, or overdue for payment, simply make a manual payment for this with the new card and this will become the stored card for future payments.
If you don't have an invoice due, login to your portal and from the Billing menu, select the option to Update Payment Card. You can then enter your new details. We will validate 50p on the card and this will be voided off immediately. You'll need to ensure that you have 50p on your balance otherwise the change request will fail.