Updated June 6th, 2019.
You can return just about anything to Apple if you do it within 14 days. So, if you bought an Apple TV last week, and Apple introduces a new Apple TV Monday at the 2015 WWDC, you can bring your old one back and buy a new one. If you bought an Apple Watch 38mm, found yourself squinting at its tiny type, and wishing you had the 42mm version, you can bring it back and get the larger one. Even if you simply changed your mind and would rather get your money back, that is fine with Apple. It’s about as easy as returning something gets.
The official policy is “14 days.” If you’re slightly past that, call 800-692-7753 (800-MY-APPLE), explain the situation, and see what they can do for you. It’s worked for me.
Here are the details.