This was very helpful from from another thread:
2. Make sure you know what COM port your adapter is using; you'll need to know this for the first part of the update. You can see this by expanding the entry under Control Panel > Device Manager > Ports (COM & LPT). Mine was COM4 FWIW.
3. Make sure the baud rate that the first firmware update program uses (57600 specified by JVC) and the baud rate specified by your adapter's settings match. Again, go to Control Panel > Device Manager > Ports (COM & LPT), expand the tree, right-click on your device, click "Properties," click the "Port Settings" tab, then select 57600 from the "Bits per second" drop down (thanks Seegs!).
4. Don't forget to install the drivers for your adapter!
Not sure I'm remembering this exactly, but one glitch that threw me for a loop was a step where you choose a cryptic number per the instructions and it didn't seem to work, but when exiting the popup form it had actually worked.