Download via App Store and select restart
1st installation step begins - just a couple minutes
Reboot 1 - this is probably a BIOS update for my MacBook Pro
Reboot 2 - don't know what this progress bar is for
Reboot 3 - completing installation - at the 90% mark, it says 12 min, but finished in about 5 min
Login - took a few seconds longer than normal, but no need to log back into iCloud
Basically, don't worry if you see multiple reboots and the last progress bar looking stuck around 90%.