To delete a saved card, first sign in to your online account, go to 'Pay Balance' page, or add something to the basket, and proceed to payment. Once the payment page appears, there will be the option to delete (this will show next to the CVV box) Click on Delete, and then sign out of the account. 

If there are multiple cards saved, highlight the blue check button (this will be to the right of where it states Visa/Mastercard) and delete as above.