As we take payment when an order is placed, there is no way to change the payment method used on an open order. If you do want to change the payment method used, you will need to cancel the order for an immediate refund, then re-place the order.

If you have chosen to store your payment details under your account for future orders, you can change these stored details. 

To do this, log in to your profile.

From here you can add or remove payment cards at the bottom of the page.