What is the process for loading the factory firmware back onto a ubiquiiti device?

When I try to upgrade the firmware through the mesh GUI it fails.

Google search "UBNT Firmware Recovery" and click on the link titled "Firmware Recovery - Ubiquiti Wiki"

Outch! That would be quite the climb. This is straight from the OpenWRT Wiki:

Restore Original AirOS firmware on Nanostation
To restore original firmware back you can't use mtd or sysupgrade but you have to use tftp method.
First put Nanostation in recovery mode via reset button on power on, and then flash original flash image via tftp.
Source: http://wiki.openwrt.org/toh/ubiquiti/start

Assuming you have a certain UBNT POE adapter, you could try a remote reset through the POE adapter:
http://community.ubnt.com/t5/airMAX-General-Discussion/Remote-hardware-reset-circuitry-of-NanoStation-M/td-p/58320
http://wiki.openwrt.org/toh/ubiquiti/airmaxm#remote_reset

Exactly!

I would recommend buying the Ubiquiti 24V PoE adapters. Most have a reset button on the bottom of the power adapter that mimicks the physical reset button on the Nanostation. Very handy! D.

Assuming you have bbhn on your node currently use the sysupgrade version of the firmware to update it. With a ubnt poe adapter there is a reset button on the poe adapter. I have found a toothpick works great to push it. To go back to Airos unplug the Ethernet cable going to your node, push the reset button and hold it. While continuing to hold the reset button plug the ethernet cable back in. Hold the reset button for about 20 seconds and release it. You should be able to tftp thefirmware to the node. Follow the instructions above for setting the ip address on your computer. Some windows computers have trouble with identifying the network. I have found this is much easier to do in linux.