If the firmware version of your unit is Ver.1.08, you don't need to update the firmware.
Please read the manual to check the firmware version of your RMX-500 and how to update the firmware.
Unzip the downloaded file "RMX-500_v108_MAC64.zip". This file will appear: RMX-500_v108_MAC64.dmg
Notice to macOS Big Sur users
Important notice for updating firmware on macOS Big Sur