There is an issue with github, when downloading the firmware.
If you right click on the firmware file in the folder and select "save link as" you get an approx. 30kb file which is obviously not correct. I've done it foolishly 3 times, and flashed it and had to do the hard reset option.
Instead left click the file, github will then display a grey box saying "view raw", and a button on the right "download". A left click there will get you the full 260kb file.
Before the move to auto update from DWC (essentially OTA update) I think some sort of MD5 check or something like that would be a lifesaver and its quick enough to do, and common practice with things like smartphone Roms, where the consequences of flashing a corrupt file can be permanently bricking your device. I accept this is not a risk with a corrupt firmware on this device.
I checked the wiki and the instructions there are clear.
Might there also be an argument to host the firmware files on the duet3d.com site somewhere with slightly more conventional download behaviour, i.e. right click would work etc…