Try this:
1. Press the Open button on your remote control to open the drive tray
2. Press the Setup button on your remote control
3. Press the Next button on your remote control
4. Press the Next button on your remote control
5. Press the Next button on your remote control
6. Press the Next button on your remote control
7. The currently selected region should be displayed on the onscreen display
8. Using the Left button on your remote control, change the region to 1 for region 1, 2 for region 2, and so on, or 255 for multi-region playback
9. Press the Setup button on your remote control
Credit to bubblegum for this!