I told AppStore to install this. When I restarted, there was the usual login display of accounts. I selected one and entered my password. The progress bar paused halfway across for some seconds, then the the screen did something too fact to see and progress continued. Then a full screen log was displayed, with the notice that the installation had failed and several choices. I chose restart, same thing. I used the power button to shut down, same thing. I was able to work by starting with Option down and selecting the my system drive. This continued for a couple of days. Then yesterday the machine started from the system drive.
AppStore is showing this update under Update Available. I get notifications that there is an update, offering to restart.