Hi
Subject to its return policies, Apple only accepts products for exchange or refund that have been purchased directly from Apple.
For an Apple Watch that has been purchased directly from Apple, Apple's Standard Return Policy varies by country, but is typically 14 days.
More information and instructions are available here (for the US, for example):
Returns & Refunds - Shopping Help - Apple
If you are in a different country, then check the corresponding page for your location.
Other retailers' return policies may vary.