The current Firmware Version for the Pro Model is available here:
Important! If you are trying to upgrade from firmware version 4.3 and you do not first to a factory reset, it will probably brick your device! If you are upgrading from firmware version 4.3 to any newer version, you will first need to do a factory reset on your device in order for the new firmware to update.
You can tell which version of the firmware you are running by logging into the web based admin interface and scrolling down to the bottom of any page.
To do a factory reset, log in to the admin interface and click the dropdown menu named system, and then click on Firmware. In that page , click the button that says reset to factory defaults. You will be prompted one time to confirm the action.
After you have done a factory reset you should be able to upgrade to any newer version of the firmware.
This only applies to firmware version 4.3. It is a result of the organization of the filesystem. In newer firmware versions, the kernel modules are offloaded into /etc/modules.d to keep them modular and organized.
Once you have done a factory reset you can follow the normal upgrade instructions at the link above.
Important! If you are trying to upgrade from firmware version 4.3 and you do not first to a factory reset, it will probably brick your device!