Source: readme file inside modx251.zip updater file:

Updating Procedure

NOTICE:
All User memory (Library data, User data, User sample and so on) will be initialized by this updater.
Please make sure that you save all User data to a USB flash drive before performing this update.

Required Items
- One USB flash drive of over 1GB capacity formatted on the MODX

1. After downloading, copy the following uncompressed files to a USB flash drive that has been formatted on the MODX. No other files should be present on the USB flash drive.
8Q86OS_.PGM

2. Power off the MODX.

3. Connect the USB flash drive prepared in step 1 to the [USB TO DEVICE] terminal of the MODX. Power on the MODX while holding down the [Cursor UP] and [UTILITY] buttons until the "YAMAHA" logo appears on screen.
At this time, please do not press buttons other than [Cursor UP] or [UTILITY] buttons.

4. The MODX OS updater will begin with the message "Searching for the updater..." and "MODX updater *.**.*".
When the message "Finish. Please turn off." appears, turn off the MODX and remove the USB flash drive.

WARNING:
DO NOT power off or remove the USB flash drive from the MODX until the "Finish. Please turn off." message appears. If power is accidently interrupted while running the updater (by disconnecting the AC cord, etc.), the unit may become inoperable. If this happens, run the updater again. If the unit still does not function after doing this, please contact your nearest Yamaha service center.

5. Power on the MODX.

6. Execute the Initialize All function ([UTILITY] > [Settings] > [System]).

This completes the update process.

... I guess somewhere they could tell users to unzip and read the readme file for full instructions.