Open the Apple Watch app on your iPhone
Tap the My Watch tab
Tap All Watches
Tap the i button next to the watch you want to unpair
Tap Unpair Apple Watch
If prompted, enter your Apple ID password
Tap Unpair to confirm
Keep your iPhone and Apple Watch close until the process finishes
If you have multiple watches, repeat for each watch you want to unpair
If you can’t access the iPhone, press and hold the side button on the Apple Watch
Tap Settings
Tap General
Tap Reset
Tap Erase All Content and Settings