To downgrade the firmware on BrightSign you need to use BA: Connected. It's best to use the browser version.
Login to BSN.cloud
Go to the Network Tab
Click the gear icon for the unit you want to downgrade
Go to the Advanced Tab
In the BS OS Update Section, click Custom URL
Paste in the URL
Click Update
The unit will download the firmware from the link you pasted in and then reboot. The firmware will be installed. The unit will reboot. You can confirm the firmware version was installed in the Info Tab.