Apple watch
How do I connect it to my iPhone 12x max to my watch when it says I need updated software but it’s up to date
Apple Watch
Which series is it and which watchOS version have you got? It is also possible to update the watch directly by itself, without the iPhone:
Update your Apple Watch - Apple Support
If you happen to have a backlevel watchOS V6, then you would need to find an older iPhone with iOS 12 and try the same pairing with that first.