How to update iPhone to iOS 16?

The iPhone 6s does not support iOS 16 and has reached a max of iOS 15.8.5 as of now. Any future updates will only be security patches which come in dot updates, there will be no upgrade. You'll need an iPhone 8 at a minimum to run iOS 16.

iOS past iOS 15 is not an option for iPhone 6s.

You’ll need a newer iPhone, iPhone 8 or later for iOS 16, or preferably iPhone 11 or later or iPhone SE 2nd or newer, for support on the current iOS 26 version.