How to update my iPhone 7 Plus to iOS 16?

You can’t. Your iPhone 7 can only be updated to iOS 15.8.3, and not past that. You need an iPhone 8 or later to run iOS 16, but note that the latest iOS is now 18, and that requires an iPhone XR or later.

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.

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