Your Apple Watch is, in its own right, a tiny computer.
Just like any computer you’ve used, your wrist computer is susceptible to various software hiccups, some of which may prevent it from starting up or functioning properly. Should that happen, you’ll need to perform a full restore of the watchOS operating system.
Unfortunately, Apple has made it difficult for users to restore Apple Watch so you’ll need to schedule an appointment at your nearest Apple Store in case something goes awry with the watchOS software that couldn’t be resolved by resetting to factory settings.
While Apple Watch lacks a Lightning port, it does sport a hidden six-pin diagnostic port for service technicians inside the groove of the bottom strap connector slot. Thankfully, MFC’s unofficial iBus dongle lets you connect any Apple Watch via the standard Lightning cable to iTunes on your Mac or Windows PC, allowing you to perform a full restore.
We should point out a major caveat: Apple does not make the full watchOS restore files available publicly so you’ll need to obtain the related firmware files from other sources before attempting to restore or update your Apple Watch with iBus.

How to enter DFU mode on Apple Watch

In order to restore software on your Apple Watch from a firmware file on your computer, or downgrade to an older firmware, you’ll need to put the device in DFU mode.
1) Remove the strap on the button side of your Apple Watch case.
2) Place the sewing needle in the port hole. Don’t press the needle to access that door like you would when opening the SIM card tray on your iPhone: instead, apply light downward pressure while pole-vaulting the needle to the side in order to pop the cover off the port.
Here’s a video showing the process.
3) Now connect your iBUS dongle to the watch’s diagnostics port.
4) Connect a Lightning cable to the other end of the dongle and the other end of the cable to your computer’s USB port, then launch iTunes.
5) Turn on your Apple Watch and wait until it boots.
6) Press and hold both the Digital Crown and the Side button for ten seconds, then release the Side button while still holding the Digital Crown until iTunes greets you with a messageconfirming your Apple Watch is in DFU mode.
Not unlike putting your iPhone, iPad or iPod touch in DFU mode, you will be asked to select a watchOS firmware file (.IPSW) for restore.
The following video shows the full Apple Watch restore process with the iBUS dongle.

It’s worth noting that performing any kind of software installations through the diagnostic port on your Apple Watch will void your warranty and potentially impact the water resistance of the device so proceed at your own risk.