Manual Pair

As explained in If you can't pair an Apple Watch with your iPhone.

"Unpair Apple Watch if you have its paired iPhone

1. Open the Watch app on your iPhone, tap the My Watch tab, then tap All Watches at the top of the screen.
2. Tap the "i" icon  next to the watch that you want to unpair, then tap Unpair Apple Watch.
3. Tap again to confirm, then wait for the watch to unpair.
4. After the Apple Watch unpairs completely, you can pair again."

It's here;