Just a note to people trying this, it looks like you should use the serial number listed under the last menu option (in normal firmware). The exclamation point, when selected shows the full serial number of the device.
There is one written on the outside of the dash cam, but it seems to be a short version, this might cause issues?
Cancel that, see the post below, appears you actually want to use short version.
Edit: Also, for anyone having problems with the final step (flashing new firmware). Make sure you after you format the SD card you first boot it in the camera at least once (without the firmware file). It seems format or setup the card once again (which was overwriting my firmware file). Then, turn off the dash cam, copy the firmware file onto the SD card, connect the camera to power, and turn on. You should get a quick-flashing yellow light at that point (with screen off), which means it is updating.
Seems to be working fine after this! Note: Mine also shows the strange characters in serial number display though, but not sure this affects anything.