How to update an iPhone 7

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. 

The iPhone 7 Plus can update as far as iOS 15.7.7

But, it can't update to iOS 16, if that might be what you are looking for.