I need to exchange my loop band from a 10 to a 9 size

For returns, refunds, or exchanges see:

Returns & Refunds - Shopping Help - Apple

From that link:

• "You have 14 calendar days to return an item from the date you received it."
• "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."