Ok - success! Here's some addl info for others wanting to do this firmware upgrade:
1- download the appropriate Windows driver for Hubitat stick from link Raidflex listed earlier. In my case the Win10 link...
2 - extract the driver .zip file and run the appropriate installer app. In my case the 64 bit one named CP210xVCPInstaller_x64
3 - although Windows detects the Hubitat stick upon insertion, it does not find/assign any drivers to the 2 devices (listed in earlier post above)
4 - go into Device Manager (must be an admin) and navigate to Other Devices. You should see 2 devices that were added for the Hubitat stick: HubZ ZigBee Com Port and HubZ Z-Wave Com Port. We're interested in the 2nd one with Z-Wave. Rgt-click and choose Update Driver. Then choose Browse my computer..., then choose Let me pick from a list of available drivers on my computer. Navigate down to Ports (COM & LPT), click [Next], in the Manufacturer list find Silicon Labs, and in the Model list highlight Silicon Labs CP210x USB to UART Bridge. Tell it to install it, and ignore any compatibility warnings.
5 - now that Hubitat's stick driver is installed, you can run HomeSeer's Z-Flash utility. Once Z-Flash loads, it will have drop downs in upper left for Interface and Com Port. Choose USB Z-Wave Interface. In my case there was only one option for Com Port (COM3, but yours might be different?). Once those 2 items are selected, there is an [Enable] button just below the Interface selection, click it. That should populate the area titled "Step 2 Device Management" and list the Leviton dimmer. Put a checkmark next to dimmer in that list, then - if all the planets are aligned - you can click on Step 4's [Start Update] button. Note - you might have to first choose the appropriate file in Step 3 Select the Update File. Mine had only one choice - the version 1.20 file...
Another note: I did this on Windows 10 Professional running under Parallels on my iMac. Don't know if the iMac was at fault, but I did have to repeat Step 4 three times before it successfully updated. The first two times it got to almost 80 or 90% done (7 to 8 minutes), then reported "Device reports unable to receive, aborting".
Happy updating 
