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
If prompted, enter your Apple ID password
Wait for the unpairing process to finish
If you do not have your iPhone, erase the Apple Watch on the watch itself
On the Apple Watch, open Settings
Tap General
Tap Reset
Tap Erase All Content and Settings
Enter the passcode if prompted
Confirm the erase
Remove the Apple Watch from your Apple ID on iCloud if needed