How do I change expiration date on my credit card to update it?

I have never needed to update mine; they have always updated automatically. Also see the following from Change or remove the payment cards that you use with Apple Pay - Apple Support

Update your Apple Pay card number and expiration date

With Apple Pay, your payment card has a unique Device Account Number used to process payments and an expiration date associated with that Device Account Number.

If your card expires or you otherwise get a new card:

• The card number and expiration date linked to the Device Account Number should be updated automatically. 
• If your card issuer doesn’t support these updates, you might need to remove the card and add it again.