Apple Watch series 3 won't update to iOS 8 during set up
I was given a hand-me-down Apple Watch series 3 which was belonged to my big sister (I'm not complaining). I factory reset the watch because we forgot the passcode to unlock it. I am currently trying to set it up, pairing it with my iPhone 11 (I have already updated the iOS of my iPhone to iOS 15), but the Apple Aatch needs to be updated before I can completely finish setting up.
The problem is it won't update and it's stuck on "preparation"/"preparing" stage of the update and when I leave it alone, the apple watch somehow unpairs itself and went back to the first step of setting it up (choosing a language).
I tried searching for solutions but all of the solutions shared are when the Apple Watch has already been set up.
How can I fully update the Apple Watch series 3 with my iPhone 11 while the Apple Watch is setting up?
Apple Watch