You can manage your payment methods in account settings under Payments. There you can add a new card (by making a payment and choosing to save it), remove a saved card, or set which card is your default. The default is used when you choose "Pay with default card" at checkout.
To set a different card as default, open Payments and use the "Set default" option on the card you want. Only one card can be default at a time. If a card has expired or you no longer use it, remove it so your list stays up to date. You can always add a new card at your next checkout.