MacBook Air didn’t download the system update successfully, and hang in the mid. Pls adv how to go back to normal OS MacOS could not be installed in your computer 
The installer resource were not found
Quit the installer to restart the computer and try again

Pressed restart then it hang and get this MacOS message again
This worked for me (could probably be streamlined). The goal is to remove macOS' files related to failing install attempt.


Booted into to 'Safe Mode' by holding down shift while restarting.
Opened a Terminal (/Applications/Utilities/Terminal.app)
Deleted /Volumes/Macintosh HD/macOS Install Data as described here. I needed to use sudo and rm -rf so be careful. (Could also be done from macOS Recovery: see below, but Volume names may differ).
Rebooted and got only a question mark. Uh Oh.
Booted into macOS Recovery by holding down Command-R during reboot and selected the start-up disk to boot from.
Restarted and ALL OKAY!


Will try the update again when feeling braver.

UPDATE: After backing up I took the cautious approach and downloaded the full combo update from https://support.apple.com/en_GB/downloads, manually killed any app that might mess with shutdown and ran the update package. It all worked fine this time around.