You can upgrade your Konftel 300Mx with a SD card.
Download the SD file from www.konftel.com/Support/Upgrade.
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.