Update iPhone 6s Plus software from version 15.6

An iPhone 6s, iPhone 6s Plus, iPhone 7, iPhone 7 Plus, or iPhone SE 1 isn't capable of 16 or above; they require an A11 or better CPU. To install 15.8.6, use a computer or the Software Update function in Settings.

(262638)

An iPhone 6s Plus can be updated to iOS 15.8.5 and no higher. If you need a higher iOS version, then you'll need retire your very old, out of date iPhone and buy a new iPhone.