I bought a used Lytro Illum with firmware 1.0.1 (15).

These instructions said to just put the last firmware version file ILLUM-update-2.0.0 (42)-u.lfu on the SD card and turn on the camera with that SD card.

However this worked best for me:

1. Reformat the SD card from the camera.
2. Take one picture.
3. Turn off the camera.
4. Remove the SD card and place it in a PC.
   1. I used an OpenBSD system for this.
5. Place the .lfu firmware file in:
   1. The root of the SD card.
   2. The DCIM directory.
   3. The DCIM\100PHOTO directory.
   4. The LOST.DIR directory.
   5. The MISC directory.
6. Unmount/eject the SD card.
7. Put the SD card into the Illum.
8. Start the Illum.
9. It should now find a firmware update and suggest to update the firmware.
10. Update the firmware.

With these steps I updated:

1. 1.0.1 (15) to 1.0.2 (19)
2. 1.0.2 (19) to 1.1.0 (31)
3. 1.1.0 (31) to 2.0.0 (42)

I probably should have been able to go from 1.0.1 (15) to 2.0.0 (42) with above steps, but by the time I had these steps figured out this was the upgrade route I had taken.