My iPhone 7 download software but failed to install

Since you do not indicate what happens when you attempt to install it, whether you receive an error message or not, here is a support document for problems with unable to update, If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support

What is the exact problem and at what step it fails? What error do you get in iTunes?

p.s. If you get Error (-1) check this article, it's a baseband chip: iPhone 7 Repair Program for "No Service" Issues - Apple Support

Roboot your phone and try again. On iPhone 7 press and hold the volume down and power buttons at the same time (keep holding past the swipe to shut down) until the Apple Logo appears, then let go of both buttons. Your phone will restart. See if you can update once it comes back up.