How to get update 16.3.1 on iphone 7 plus

Not possible to install iOS 16. The best that the iPhone 7 Plus can do is iOS 15.7.3

Unfortunately, that is not possible.

That model can be updated to iOS 15.7.3, no further.

That is not possible.

The iPhone 7+, released by Apple 7½ years ago, cannot use iOS 16.

It’s too old.

The non-technical answer is that the phone is too old. A more technical answer is that the hardware in the iPhone 7 cannot meet the demands of the software in iOS 16.