Copied from someone else - I didn't write 99% of what is below but I think it's very useful - especially if you are messing with json files on your UM3 and it stops booting.
you'll find a recovery image here: recovery.img.
You can install this update by putting this image in a micro sd card - but don't just copy the file there you have to do a special process to make it a bootable sd card.
here is a nice guide on putting a disk image on a sd card:
If you are working with MacOS, you can better use ApplePie baker:
• Insert the Micro SD card in your computer.
• Use the IMG recipe section and search for the file.
• Click on the Restore Backup button.
• Once that is done you can put the SD card back in the printer and check if it boots correctly.
Then, removing the plastic cap in front of the olimex board (it is located on the bottom of your printer), find the SD card slot and insert the SD card:
The SD card slot is located on the smaller board in the back. (Where the network cable connects into)
In order to reach it you will need to remove the micro USB cable in front of it
Now, booting up (only once) with the sd card inserted, wait until a green light goes on on the led ring around the control knob.
The display will also tell you to remove the sd card and reboot.
After all this you probably want to update to the latest firmware as I'm not sure that the link above will always be the latest but the printer will know if it is the latest or not.
Please message me with more details that you learn as I have never tried this with an Olimex board (I've done the same basic procedure on a Beagle Bone Black).