Apple Pay will not let me update my exp date on my new cards

Mine have alway updated automatically but if you need to change one, see the following from Change or remove the payment cards that you use with Apple Pay - Apple Support

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.