Contact Elko support for the latest version of the thermostat firmware.
The version 1.2.6R package contained:
ap1.bin
avrboot.bin
elko_termo_og_boot.BAT
I bought a SanDisk 128GB SD card and formatted for FAT 32
using Disk Utility
(macOS). I unzipped the package and copied the files over to the SD card at the command-line. I am using the command-line to avoid any magic the GUI versions might introduce. I also rm'ed any hidden directories and files introduced by the OS. Unmount/eject the SD card.
The upgrade procedure is quite simple actually. As it states on the back of the thermostat: