How to update my iPhone 6s software?

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 15.8.3, use a computer or the Software Update function in Settings.

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iOS 15.8.3 is the end of the road for the iPhone 6s. You likely already have this installed.

To check.....Tap Settings > General > Software Update.