Unpairing an Apple Watch restores it to factory settings. You may want want to do this before pairing to a new device or transferring ownership of your smartwatch.
Launch the Watch app from your Springboard
Tap All Watches at the top left of the My Watch tab
Press the i icon to the right of the Apple Watch you would like to unpair.
Tap Unpair Apple Watch, then press the Unpair Apple Watch button to confirm.
Enter your Apple ID password then tap the Unpair button at the top right to continue.
The unpairing process will take a minute or two to complete.
You'll then be ready to pair the Apple Watch with a new device.