I want to update my iOS

Use a computer or the Software Update function to install 15.8. If you encounter a specific issue except as noted below, describe it in this thread.

An unit on 12.4.2 through 12.5.7 is an iPhone 5s, 6, or 6 Plus, not an iPhone 6s, and isn't capable of 13 or above.

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iPhone 6s cannot update to iOS 16 or iOS 17 if that is what you are asking. The iPhone 6s is too old.