Keep your iPhone and Apple Watch close together
Open the Watch app on your iPhone
Tap All Watches
Tap the info button next to your Apple Watch
Tap Unpair Apple Watch
Enter your Apple ID password if prompted
Choose whether to keep or remove your cellular plan
Wait for the unpairing process to finish
If you do not have your iPhone, erase the Apple Watch from Settings on the watch
On the Apple Watch, open Settings
Tap General
Tap Reset
Tap Erase All Content and Settings