You can upgrade your Konftel 300Mx with a SD card. 

Download the SD file from

Put that in the root of the SD card and then put the Konftel conference unit in upgrade mode (MENU > SETTINGS > UPGRADE MODE). 

Insert the SD card and press the HOLD button to read from it. 

Follow the instructions on the display on the Konftel. 

* Please note that when upgrading using SD card it will reset the phone to factory defaults.