To delete the card number that is saved from previous placed order payment, place a item in the cart and proceed to the payment step.
Choose the card as the payment method and you can see,
"Reset card Info" then you can Reset the card info saved by ticking "X"
(X in the next card number tiny box “Save this card Info as my default card)
You don't need to proceed with the order process completely and just delete the item or cancel the order.