After the system reboots successfully:
Before any firmware change, backup your configuration. In PCPro: nec sl1000 firmware upgrade
If the upgrade corrupts the database, you can restore from this backup after the new firmware is loaded (though note that major version jumps may require database conversion). After the system reboots successfully:
If the system fails to boot entirely (all LEDs flashing error code): Update PCPro: Ensure your PCPro software version matches
This is advanced and should only be attempted by a certified NEC technician.