How can I update my iPhone 6s to iOS 16.0?

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.

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The minimum requirement to run iOS 16 would be an iPhone 8, iPhone 8 Plus or iPhone X.

You cannot update an iPhone 6s to iOS 16 or above. iOS 16 is the end of the line.