Windows

1. Install the latest version of iTunes from the Microsoft Store.
2. Plug your iPhone 7 Plus (Global) in to your PC.
3. Open iTunes, and navigate to your iPhone 7 Plus (Global).
4. Hold the Shift key on your keyboard, and press "Restore".
5. Select the iPhone_5.5_P3_15.8_19H370_Restore.ipsw file that you downloaded.
6. Wait for the restore to complete, and the iPhone 7 Plus (Global) to reboot.

macOS

1. Plug your iPhone 7 Plus (Global) in to your Mac.
2. Navigate to your iPhone 7 Plus (Global) in Finder.
3. Hold the Option or Alt key on your keyboard, and press "Restore".
4. Select the iPhone_5.5_P3_15.8_19H370_Restore.ipsw file that you downloaded.
5. Wait for the restore to complete, and the iPhone 7 Plus (Global) to reboot.

Linux

1. Install idevicerestore.
2. Plug your iPhone 7 Plus (Global) in to your PC.
3. Open a terminal, and navigate to the IPSW file that you downloaded, e.g. cd ~/Downloads
4. To perform a full erase restore, type idevicerestore -e iPhone_5.5_P3_15.8_19H370_Restore.ipsw and press enter.
5. Wait for the restore to complete, and the iPhone 7 Plus (Global) to reboot.