How to get iOS 16 updatefor my iPhone 7 Plus?

If it's an iPhone 7 Plus then you are out of luck.

iOS 15.8.3 is the most current it will take, probably because it has the Apple A10 Fusion chip, and the next version up iOS 16.7.10 is for the Apple A11 Bionic chip.

You cannot update an iPhone 7 Plus to iOS 16. The iPhone 7 Pus originally ran iOS 10 and can be updated to iOS 15. For it, iOS 15 is the end of the line.

I believe that all of the iPhones that can run IOS 16 have Neural Engines. The Neural Engine first appeared in the A11 Bionic chip that Apple used in the iPhone 8, 8 Plus, and X.

you can't

end of the line for iphone 7 is

Current: iOS 15.8.3, released July 29, 2024[3]

iPhone 7 - Wikipedia