For my iwatch to work it say update to iOS 16 and I’m on iOS15. But I’m on iOS 15.7

iOS 16.3 is available:

About iOS 16 Updates - Apple Support

for these iPhones:i

OS 16 and iPadOS 16 are compatible with these devices - Apple Support

I am afraid an iPhone 6s (according to your tag) will only update to iOS 15.7. If you have one of the newest watch models that require iOS 16, you will need a newer iPhone model. I am afraid there is now other workaround.

It isn't possible to pair a watch on 9.0 or above with an iPhone 6s.

(233600)