UPDATE via WEB GUI
1) Go to http://192.168.1.200/ or your specified IP address
2) Backup the current configuration by choosing the "Download Settings from Device" button and save the *.json file.
3) Navigate to the "File Systems"(previously called "Files") section from the main menu.
4) Select "Firmware"(previously called "Boot Partition") and click the "Upload file" button.
6) Upload mgb4-install*.tgz file.
6) Upload mgb4-flash file (blank txt file with no extension).
7) Optionally, upload the mgb4-keep file to keep the current configuration.
8) Power off the FG4 MultiBox by pressing the ON/OFF button once. Wait until the dimmed blue LED status occurs.
9) Power on the FG4 MultiBox.
10) The FG4 MultiBox boots into the flashing process.

UPDATE via SDCARD (keeping configuration is not possible)
1) Power off the FG4 MultiBox.
2) Copy the mgb4-install*.tgz to the SDCARD.
3) Create the mgb4-flash file on the SDCARD.
4) Insert the SDCARD into the FG4 MultiBox.
5) Power on the FG4 MultiBox.
6) The FG4 MultiBox boots into the flashing process.
7) The FG4 MultiBox powers off itself after the flashing is done.

For more update options see user manual.