I have a MacBook, and the USB-C Multiport Adapter was connected to it to charge the computer. After I installed the latest OS X update, I saw this:
Clicking Install, I then saw the following description for the update:
So if you have a Macbook, and have one of these adapters, plug it into your laptop to apply the update to the adapter. You’ll need to restart your MacBook after applying the update.
If you’re not seeing the updater, or if you have more than one adapter and need to run it again, you can find it in /System/Library/CoreServices/Firmware Updates/USB-C Multiport\ Adapter/. (H/t to @pmod for pointing out the location of the updater.)