There are a number of "gotchas" here and some are probably not unique to this update. I almost bricked my camera last time (a6000 a few months ago) and this time (a6500) I had to try several times to get it to work. As much as it pains me to say it, if you have a Windows PC available, use it for this update. For MacOS X High Sierra, here are my tidbits of advice in no particular order..

1. Set your computer to NEVER go to sleep nor to let the monitor sleep. You can restore your preferences later but DO NOT allow the computer to sleep when updating your camera. Also, eject all your USB drives before you start the process; the updater messed with mine and caused endless messages about not properly ejecting (I did not mean to eject them). Close all the apps that you can so they cannot interrupt the update. I can't prove it's needed but this would be a good time to do a full, power-down boot to ensure the OS is fresh.

2. The installer wants you to ensure your USB connection is set to "mass storage" and for you to remove the "memory card".

3. This update and probably any Sony update for a while needs a helper app which is on the support page for the update but is not as obvious as it needs to be. If you are running High Sierra you must run the helper app first and keep it running but that is circular as you will see below.

4. Download and install BOTH apps. The regular installer has some cautions about doing some things before you connect the camera to the computer but the helper app wants you to make the connection in order to run and the helper app must be running in order for the update app to work. So my solution was to do all the things in the first step on the helper app (camera settings) then close it, then start the helper app then while it is running, re-start the updater. Clear as mud!
-------------------------------------------------
I did eventually succeed without hurting my camera. I note in passing that the vague reference to camera stability is the only reason I ran the update since I don't have the lens for which is is a necessity. I wasn't having any problem with the camera so in hindsight I might have let it go. That said, it has been my experience with all kinds of computers (the camera is a computer) that updates to firmware or software are best done in the intended increments. Wisdom.

I blame Sony rather than Apple but no matter who I blame, updating my Sony cameras on my Mac computer is really not a happy experience. The time I almost bricked the camera was worse but this was pretty anxiety producing because whenever it did not work (the first few times) it reminded me of the bad experience on the other camera. Don't mind me - I'm just venting.

I hope I don't end up hurting anyone with this advice but right now I feel obligated to tell all the Sony&Mac Hoggers, just in case.