How can I unpair the Apple Watch from my old iPhone which I no longer have?

Read here => Unpair and erase your Apple Watch - Apple Support

Simply erase the watch. Push the Digital Crown, Select Settings (the cog wheel) and scroll to the bottom of the list for Reset.

Note you will also need to login to your Apple ID account (iCloud) and remove the watch from your devices to release Activation Lock if you wish to sell the watch or re-pair it to a new iPhone.