Can I update my iPhone 6s Plus to iOS 16?

The iPhone 6s can only be updated as far as iOS 15.7.5. If you have not installed this yet, why not?

If you want to run iOS 16, you will need an iPhone 8 or higher.

No. An iPhone 6s isn't capable of 16 or above; they require an A11 or better CPU.

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You should be using iOS 15.7.X. Obtain it as follows:

Update your iPhone or iPad - Apple Support