How do I update my iPhone 6s?

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

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Tap Settings > General > Software Update

If the message indicates that your iOS version is up to date, there are no updates for your iPhone

If you see a message that a new update is available, tap to update the software on your phone to iOS 15.8.6