A direct quote from those instructions I suggested that you read:
"As noted above, there are two different versions of CPU firmware for the
PSR-500/PSR-600 scanning receivers. To determine which version you should
use, turn your radio off, then turn it back on while holding down the PGM key.
Refer to the “uP Boot Ver:” line at the top of the display to determine the Boot
Version used in your radio.
BootF10F11_U2.1.exe with Boot Versions prior to 1.4. Use