Did you do exactly the way it's written in "chapter" 3 when card placed in keyboard?
The part about repeat press buttons?

... Now copy the file "KN24PRG.DAT" to the SD card.
3. The update on the KN2600:
- Insert the SD card into the SD slot of your KN2600.
- Press and hold the first 4 Panel Memory buttons and
turn on the keyboard.

- Select UPDATE repeatedly until the process starts.
- At the message "Completed !! Turn on AGAIN" switch off your KN2600 and
after about 2-3 seconds again.

I've downloaded the update file from CA and added here as well if you want to try with a new zip just in case your file is damaged in any way.

kn26-11.zip (6 downloads)

Cheers 🥂
"Success is not counted by how high you have climbed
but by how many you brought with you." (Wil Rose)