There is an area in the ROM that is not user accessible that checks for good Firmware before proceeding with the loading of the Firmware data. Therefore, if you follow n4yek good suggestions the scanner should come up in the state needed to update the Firmware, but as n4yek said you must follow the instructions EXACTLY, Do Not Skip any Steps.
Unless the radio has a physical problem (which you cannot cause AFAIK by trying to do a Firmware update) it is not suppose to be possible to "Brick" the radio by making a mistake in updating the Firmware.