If you were to simply do a clean install, the latest “official” iOS version will be installed onto your device. If you had the .ipsw file for a specific iOS version (iOS 11, for example), that is the version that would get installed, provided you direct iTunes to install it. With that in mind, my definition of a clean install is putting the device in DFU mode so that iTunes can download and install a fresh full copy of iOS. After the installation of iOS, you can choose to restore from a backup or set it up as a new device. Since you’ve had iTunes install a fresh copy of iOS, why risk corrupting it with a backup? I recommend you set up the device as new.
Directions:
1. Put the device in DFU mode. DFU = Device Firmware Upgrade.
https://www.theiphonewiki.com/wiki/DFU_Mode
2. Connect the device to your computer and then initiate the computer’s iTunes app, preferably the latest version.
3. When iTunes detect the device in “recovery”, select one of the following:
A. If you simply want to do a clean install of the latest official iOS version for your device, select “restore and update”.
B. If you’ve downloaded the .ipsw file for a specific iOS version, select CTRL + restore or OPTION + restore (I forget which one) and then direct iTunes to the .ipsw file to be installed.
4. When the process is complete, you will be met with the welcome screen. Simply go through the prompts like you did when you set up your device for the very first time.