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
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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.