How to update iPhone 7 Plus to iOS 15?

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. 

Make sure that you have a good strong Wi-Fi Internet connection and that the battery is fully charged.

Tap Settings > General > Software Update.