Hi
For an Apple Watch that has been purchased directly from Apple, Apple's return policy varies by country, but is typically 14 days (holiday return periods can also apply). Other retailers' return policies may vary.
Apple currently offers a trade-in service for Apple Watch in certain countries.
Follow the links here (for the US, for example) for more information and to get an estimate of the trade-in value, based on your Apple Watch model and its condition:
If you are in a different country, then visit the corresponding page for your country instead, if available.
You may otherwise wish to consider selling your Apple Watch privately. If so, be sure to follow Apple's instructions for unpairing your Apple Watch and turning off Activation Lock, such that it is left ready for a new user to set up as their own: