Check out the video demonstration posted
by Alireza via Steve Troughton-Smith on YouTube how he
restore Apple Watch firmware with iBus tool using iTunes on macOS.
Alireza restores watchOS on an Apple
Watch using iBus tool with step by step.
As, Apple Watch lacks a Lightning
connector, but the iBus dongle connects to its hidden diagnostics port that
provides Lightning-to-USB connectivity on Apple Watch.
This iBus tool connects your Apple Watch with
Lightning USB cable to your computer’s USB port for restore & upgrade and
repair software etc. 

With iBus, you can now restore or upgrade Apple Watch watchOS using firmware file in iTunes for Mac or Windows same like iPhone or iPad
iBus supports both 38mm and 42mm sizes.
Simply connect iBus to your watch’s hidden port and plug in iPhone’s
Lightning cable to the one end of the dongle, and other to the USB port of your
Mac or computer.
With iBus, you can now restore or
upgrade Apple Watch watchOS using firmware file in iTunes for Mac or Windows
same like iPhone or iPad. Along with that you can also downgrade your Apple
Watch watchOS to previous version with iBUs.

Take a look at the video below on how to Restore watchOS using iBus and iTunes;

Price of iBus dongle varies for Apple Watch generation where $100 for the first-generation Apple watches  and $120 for Series 2 Apple Watch models.