If you want to erase everything (all data) in your Apple watch, then follow the simple steps below on how to factory reset your Apple Watch.
When you’re ready to begin the reset, get the Apple Watch you wish to erase, then do the following:
1. Open the “Settings” app on the Apple Watch and go to “General”
2. Scroll down and choose “Reset”
3.Tap on “Erase All Content and Settings”
4. Enter the passcode for the Apple Watch
5. Scroll down and confirm you wish to reset the Apple Watch and erase all data from the device
Once you confirm the reset, a spinning wait indicator will appear on the screen for quite some time, eventually followed by an Apple logo with an indicator around that.