To pay with another debit or credit card, just click on 'Add a new card' below your list of saved cards on the final step of setting up your transfer, 'How would you like to pay?'.
You can then enter the new details.
To remove saved cards, you can click 'Manage Cards'. You'll then be able to remove the details of any expired cards or cards you don't want to use any more. You can also do it in Settings, just choose "Your saved bank cards".