How to unpair watch from lost phone

Put your Apple Watch on its charger

Press and hold the side button as if you were going to power it down

Do a firm press on the POWER OFF slider and let go

Tap Erase all content and settings

Alternately, you can go to Settings/General/Reset to erase and unpair your Apple Watch if you know your passcode, but the option above is faster.

Or;

Log on to icloud.com on a computer and remove your Apple Watch under Account Settings/My Devices.