You can check Apple's support document for more details on eligible devices, but to summarize, the company will repair these models for free: Watch Series 2 sold between September 2016 and September 2017, Watch Nike+ Series 2 sold from October 2016 to October 2017, Watch Series 3 GPS and GPS+Cellular sold from September 2017 to September 2019, and Watch Nike+ Series 3 GPS and GPS+Cellular sold between October 2017 and September 2019.

You can either arrange to mail in your device by contacting Apple Support or walk into an Apple Authorized Service Provider or retail store. Take note that your device will be sent to an Apple Repair Center whichever method you choose, because the company has to verify that it's eligible for the program.