Downgrade Apple watch2

My watch2 is on watchOS 6.2.8 and I’d like to offer it to my nephew who has an iPhone 6.

I tried to connect the watch but it says to upgrade the iPhone iOS. But the phone is upgraded to the maxi iOS. (12.4.8)

I think that the only solution is to downgrade the watch OS.

How to downgrade, please. Or is there another solution to use this watch on a iPhone 6 ?