How do I install iOS 16 on my iPhone 11

It's no longer possible to install 16 on an unit capable of 17. To install 17.6.1, use a computer or the Software Update function in Settings.

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In about a week, your iPhone 11 might offer you a choice between iOS 17 and iOS 18. This window during which you have a choice of versions – if there is one – won't last very long. It will close, and then iOS 18 will be the only major version update you will be offered.

I'm basing this on what happened after iOS 17 came out, when there was a brief window during which a phone that was compatible with iOS 16 and 17 could be updated to either one of them.