Firmware for connected equipment can be updated using the Victron Connect App. It doesnt matter if you are connecting via Bluetooth or directly to a computer via the VE Direct to USB interface. That means you can use a PC, Mac, Android or IOS device to do the updating. The files are stored within the App so you dont have to have an internet connection to do the updating, you just have to have the lastest version of the app. Note that this may not apply the the VE Bus Bluetooth dongle. I dont think you will be able to update the firmware of a Multi Plus or Quattro using this method.
For further details please read the original Blog Post on the Victron website