How do I resolve iPhone 7 software update failure?

Before updating check the storage availability in the device

Check your internet connection ,remove the wifi and re add it .

once done restart the device and do update .

However iPhone 7 has become obsolete ,this device will update to OS version 15.1

You can update an iPhone 7 to the latest release of iOS 15, currently iOS 15.8.3.

You cannot update an iPhone 7 to iOS 16 or higher.

About iOS 15 Updates - Apple Support

Update your iPhone or iPad - Apple Support

Updating iOS: common issues and solutions - Apple Community