For an Apple Watch that has been purchased directly from Apple, Apple's return policy varies by country, but is typically 14 days. Other retailers' return policies may vary.
Apple does not currently offer either a trade-in program or an upgrade program for Apple Watch.
You may wish to consider selling your Apple Watch privately and putting the proceeds towards your new purchase.
If doing 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: