Plug it into the M-400's USB stick port and selct SYSTEM > USB MEMORY > FORMAT > FORMAT. If the stick fails to format, try another stick.
Once your USB stick is formatted, plug it into your computer and copy the M400.PRG file into the root directory of your USB stick. (The root directory is the primary directory such as E:\ or F:, etc - do not hide the update file inside any of the folders).
Once the update file is copied over, eject the USB stick from your computer, plug it into the M-400's USB stick port and select SYSTEM > SYSTEM UPDATE > SYSTEM UPDATE. Wait for the update process to complete.
When it is complete, you will prompted to turn off your M-400. Turn it off, then back on.
When it reboots, select SYSTEM and verify that SYSTEM VERSION number matches the firmware update number you just installed. You can now safely delete the M400.PRG file from your USB stick (using your computer) if you desire.