Never turn the keyboard on or off with a USB flash drive inserted in the USB TO DEVICE port.
If you have a USB flash drive in the USB TO DEVICE port, remove it.
If the keyboard is on, turn it off and wait for about 10 or 15 seconds to make sure no residual power is flowing through it.
Hold down the rightmost white key and three rightmost black keys, then turn on the keyboard while keeping those four keys held down, to do the "flash clear" operation. (See page 39 in the owner's manual.)
When that procedure finishes, turn the keyboard off again and wait about 10 or 15 seconds.
Hold down the rightmost white key by itself, then turn on the keyboard while keeping that key held down, to do the "backup clear" operation.
If the keyboard is not working again after that procedure finishes, turn the keyboard off again and wait about 10 or 15 seconds.
Hold down the MEMORY button, then turn on the keyboard while keeping that button held down.
If the keyboard is still displaying "Update" after performing these procedures, I don't know what else to suggest except to have the keyboard looked at by a certified Yamaha service technician.
Note that at least one user on this forum has reported getting this "Update" message on one of the PSR-E2xx models, which have no USB TO DEVICE or USB TO HOST ports, meaning there is no way to load external style files into those models. Consequently, I would tend to doubt that this message was caused in your case by loading external styles from a USB flash drive-- but you never know.