How to update iOS update iPhone 7 Plus

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support If the above Apple articles do not help, please post back with relevant information. 

You can update an iPhone 7 model iPhone to iOS 15.8.1.

Here is how you can if you aren't update that high --> Update your iPhone or iPad - Apple Support

If you are looking to go higher than iOS 15, that is not possible.

Go to settings - general - software update and start there.