If you're on a PC, use MIDIOX.
UPDATING THE OS FROM WINDOWS USING MIDI-OX
Before starting, close all other audio or MIDI or DAW software, and disconnect all other MIDI devices. If necessary, download and install MIDI-OX.
http://www.midiox.com/To prepare the Prophet Rev2 to receive system exclusive messages:
1. Press the Globals button to enter the Global menu.
2. Scroll to "10. MIDI SysEx Cable".
3. Change the MIDI SysEx cable to select either MIDI DIN or USB. If you’re using USB to update the OS, choose “USB.” If you’re using a MIDI interface, choose "MIDI".
4. Press the Globals button twice to exit.
Connect your computer to the Prophet Rev2 using either a USB cable or a MIDI interface.
If using USB, the Prophet Rev2 is a Class Compliant USB device. That means it does not require any additional drivers to be installed to communicate with your computer. It will appear in MIDI-OX and other MIDI applications as a MIDI port named “Prophet Rev2” (Under Windows XP, it will appear as “USB Audio Device.”)
If using a MIDI interface, connect the computer’s MIDI out to the MIDI in of the Prophet Rev2. Depending on the type of interface, you may also need a MIDI cable.
Most MIDI interfaces or sound cards with built-in MIDI interfaces will work. However, we have received reports that Digidesign/Avid interfaces and Native Instruments Maschine may not transmit SysEx correctly for updates. The M-Audio Uno is an inexpensive, reliable, and widely available MIDI interface for use when updating our instruments.
To load and update the OS:
Run MIDI-OX.
1. From the Options menu, choose “Configure Buffers.”
2. Set the Low Level Output Buffers “Num” and “Size” to 1024.
3. Click OK to close the window and save the settings.
4. From the Options menu, choose “MIDI Devices.”
5. In the lower left quadrant of the window, select the MIDI port to which the instrument is connected. If connected via USB, choose “Prophet Rev2” (or “USB Audio Device” under Windows XP). If connected via MIDI, choose the MIDI interface. The selected MIDI port appears in the Port Mappings window.
6. Click OK to close the window and save the settings.
7. From the View menu, choose “SysEx.”
8. From the Command Window menu, choose “Load File.” Browse to and open the Prophet Rev2 OS update file.
9. From the Command Window menu, choose “Send SysEx.”
The Prophet Rev2 displays a countdown indicating the progress of the SysEx file’s transfer. When the file transfer completes, a second, shorter countdown will start as the OS is written to memory. DO NOT power down. The unit will restart when the second countdown has completed.
The unit restarts itself after a successful update. The OS version is shown on the display then your press Global.