How do I update my iPhone 7 to latest iOS version?

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

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The iPhone 7 originally ran iOS 10. You can update it to the latest release of iOS 15.

About iOS 15 Updates - Apple Support

You cannot update it to iOS 16 or higher – let alone to the latest version, iOS 18.0.1.

it’s because iPhone 7 doesn’t update anymore