If you have a new card then it needs to replace the saved card in the system. Even though they may have the same number but different expiry dates the bank will still treat it as a new card so the old one needs to be removed
The next time you are ready to pay you will get to the page that has your saved card, below the pay now button there will be 2 links. 'Use Different Card' and 'Delete saved card and use a different card', in your case you will want to delete the saved card and use a different card. This will remove the old card and allow you to input all the details for the new card.
The other way you can remove your saved card is by clicking on your user name in the top right corner and then selecting 'My Account', this will show all your contact information as well as all your saved cards. All you need to do is click on the delete button beside the card to delete the old card. When you have deleted the old card you can then save the new card the next time you pay.