How do I update my iPhone 7 to iOS 16?

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.

(256381)

You cannot. iOS 15 is the end of the line for the iPhone 7.

How do I update my iPhone 7 to ISO 16

What you ask is not possible.