You use the GREFwTool. Set both the Scanner Type and Firmware Type to Pro-668. You can download the stock Pro-668 firmware. Put the scanner in CPU Update Mode. Upload the Pro-668 firmware file to the scanner.
If you updated your DSP firmware to 3.0, you will also need to roll that back to version 1.3 if you want to fully bring your PRO-668 back to the latest "stock" firmware released by Radio Shack. You can do that by using the Radio Shack iScan program AFTER you convert your radio back to a PRO-668. Go to "Check for DSP Firmware Update" in iScan and load the DSP version 1.3 back into your PRO-668.