How do I update expiration date

According to this support document from Apple, it should update on its own. Manage the cards that you use with Apple Pay - Apple Support

While you can't change your card number or expiration date, it should update automatically when you get a new card. If your issuer doesn't support updates, you might need to remove the card, then add it again.

I received an email saying that “There is a problem with your (my) payment information.” It was concerning an regularly scheduled payment to iCloud for storage. It told me that if I didn’t update this & my account couldn’t be charged successfully, my account would be downgraded. Also that my service might be affected. It detailed how to update my information. I think the issue is the expiration date of my credit card. I cannot figure out how to update this information. I have tried & tried. Wasted much time on it. Getting very frustrated.

See this, Change, add, or remove Apple ID payment methods - Apple Support

You're welcome.