I used the flash drive method. You download the file, copy it to the root of a flash drive, rename it as directed. With the device powered on, insert the flash drive. Power down the Flux Delta by removing the power cord, pause to reflect on what an attractive bit of engineering you are viewing, then plug in the power cord.
The Flux logo will illuminate white, then flash white and red, then return to either steady white or flashing white, I can't recall.
Because part of the firmware update involves improved (or corrected) wifi connectivity, the flash drive will be the far more reliable method.