sc2f.bin (1.55) - SHA1: 6D60E39A6C2F99FE843E59D876B03C3A3FC5FD3D

Do not update the firmware unless you already have version 1.30 or later.
Not sure why, but it'll brick.
Not sure if you need to upgrade a PLD in the unit first, and/or just flash via flashing the chips, not through the official method.
1.55 dumped ROMs are here. You don't need to update the small settings EEPROM, but I've backed up mine here just in case (if you use it, replace the serial number with yours first). Be sure to back up your original images safely before flashing any chips.

Update by connecting SIMCHECK II via serial port, ensuring you have firmware 1.30 or later, and then opening the firmware file in the SIMCHECK II Communications software. Then Send Data to SIMCHECK.
I used Windows 11 64-bit and the serial port provided on my AM5 motherboard via a pin header. The SIMCHECK Comms Software 2.10 installer doesn't work on 64-bit Windows, but the installed app does.