Where can I download the firmware update for High Sierra?

A few weeks ago I made the mistake of trying to upgrade to High Sierra. Why does Apple pull this crap without warning us? The firmware update that enables OS 10.13.(whatever) to work within a system for which it wasn't designed, to wit: any Mac that isn't equipped with a solid-state boot drive, Is a kludge. I tried to install it in an otherwise fully-functional Mac Pro 2012, and the "firmware update" alone brought down the system! I couldn't get far enough to even think about upgrading the OS, and meanwhile, my Mac Pro, upon which I depend for a living, required the services of the equivalent of a neurologist. It was out of service for close to a week. I notice that there have been at least two revisions to the High Sierra installer since that time a few weeks ago. Gee, I wonder why? Were they too proud to include a "Read Me" file with the installer? In my estimation a Read Me would have been ineffective. A skull and crossbones would have been closer to the truth. I won't be risking my livelihood to install OS 10.13 on my Mac any time soon, but If I can download the firmware update I can try it on a dummy drive.