I bought Apple Watch series 7,with magnet band with green colour,within one week band is loosing on my wrist.

Normally you have 14 days to return or exchange an item for any reason. However, items purchased at the Apple Online Store that are received between November 1, 2021 and December 25, 2021, may be returned through January 8, 2022. For more info and options, see:

Returns & Refunds - Shopping Help - Apple

You can always return the item to any U.S. Apple Store. An Apple Specialist will assist you with your return. Your refund will be processed at that time.

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.

Take it to your Apple Store and show it to them.