Hi there,
Follow these steps to unpair without having the paired iPhone
Erase Apple Watch without the paired iPhone
If you can't unpair Apple Watch because you don't have its paired iPhone, you can erase the watch.
When you erase Apple Watch instead of unpairing it, the watch is still protected by Activation Lock. Before you can pair Apple Watch again, enter the Apple ID and password that was used to set it up.
- Open the Settings app on your watch, then tap General > Reset.
- Tap Erase All Content and Settings, then tap again to confirm.
- Wait for the watch to unpair.
- After Apple Watch unpairs completely, you can pair again. You'll need the Apple ID and password that were used to set up the watch.