How can I update iOS on my iPhone 7 Plus?

Settings > General > Software Update

If you see that your iPhone is already running iOS 15.8.3, you have gone as far down the update road as you can with the iPhone 7.

There are no more updates for the iPhone 7.

Time to start thinking seriously about a new phone.

Go to settings - general - software update and see what is available. You can’t update past iOS 15.