After updating the firmware, the “UPDATE” message appears when I turn on the power.

Updating may not have been completed normally.
If the bar graph on the liquid crystal display is advancing, updating is being performed again. Wait as such until updating is completed (and the unit is set to standby).
If the bar graph is not advancing, insert the USB memory device containing the update file into the USB device insertion slot. Updating is performed again, so wait as such until updating is completed (and the unit is set to standby).