how do I upgrade my iPhone 7+

Hello,

There are different ways to ensure the highest iOS that your device will support is installed on your device.

An iPhone 6S through 7, or SE(gen. 1), will currently update to iOS 15.7.5

This support article has instructions about updating an iPhone or iPad wirelessly --> Update your iPhone or iPad - Apple Support

You can also use Mac and PC computers to update system software:

Update iOS on iPhone - Apple Support

When updating, your device will update to the highest version currently available that it will support, or it will just say it is up to date (and can't go any higher) and won't install anything. Apple indicates ( Unable to update my iPhone 6 Plus from 12… - Apple Community ) I need to point out the obvious and state that if you want to run a higher system version than your current device will run, that you need to buy a newer device.

For a list of equipment supporting iOS 16 and iPadOS 16 (and the current 16.4.1), read this article --> iOS 16 and iPadOS 16 are compatible with these devices - Apple Support

Your iPhone 7 Plus can get to iOS 15. To 15.7.5, currently.

To upgrade your iPhone: enable full-power mode, remove any add-on first-few-hops VPN client app, free up sufficient storage, remove any beta profile, force-restart, connect to Wi-Fi (and not cellular, nor hotspot), and try the update again.

Links…

Update iOS on iPhone - Apple Support

How to uninstall iOS or iPadOS beta software - Apple Support

Force restart iPhone - Apple Support

If the update fails, use a Mac or Windows PC to update.

iPhone 8 or later is required for iOS 16.

The iPhone 7+, released by Apple 6½ years ago, cannot be updated beyond iOS 15.7.5.

Update your iPhone or iPad - Apple Support