Need to update my credit card. How?

Please refer to Change, add, or remove Apple ID payment methods - Apple Support.

To update a credit card on iPhone, you will need to go into Settings > Wallet & Apple Pay. In this menu, there should be a blue button that says "Add Card". Click on this, and it should take you through the steps to add a new card. It would also be helpful to remove your old card once you are done with adding the new one.

To update a credit card on Mac, open Safari, click on Safari from the menu bar at the top of your screen, then click preferences from the menu that pops up. Once in preferences, navigate to the tab that says "Autofill". There should be some options. Look for the one that says "Credit Cards", and click on the button that says "Edit" on the right. Enter your password when it prompts you. After that, look towards the bottom left of the new menu, and click on the button that says "Add". Enter a description for the card, maybe a nickname such as "Your Name's Card". Then fill in the information required by clicking on the text and entering what is asked. After that, click done and it should be added! Like on iOS, it may be helpful to remove the old card by clicking on it and then clicking on the remove button in the bottom left.