Watch strap
My husband bought me the Apple Watch for my birthday (total amazing) but don’t like the strap will they let me change it in the shop?
Apple Watch, watchOS 4
You can try. According to:
Returns & Refunds - Shopping Help - Apple
Exchange an item
If you purchased your item from Apple in the U.S., you can take it to any U.S. Apple Store for an exchange. Item exchanges are, of course, subject to in-store product availability. It's a good idea to check if a product is in stock by visiting apple.com and then seeing if it's available for pickup at a store nearby.