Here's how:

1. Carefully open the Xbox by removing the torx screws and then releasing the clips that hold the case together. The clips and molding pieces all come off easily; pry them open carefully.

2. Load the flash firmware disc into the DVD drive and power off the Xbox.
3. Disconnect the DVD drive from the SATA cable and note the brand of drive used in your Xbox. Currently Microsoft uses Hitachi, Phillips and Toshiba DVD drives in the Xbox 360. Only the Toshiba and Hitachi drives have firmware updates.

4. Connect the DVD drive to your PC with a cross-wired SATA cable interfaced to a compatible SATA port. Confirm that the video cable is connected to the Xbox so that it won't turn off during the flash process.

5. Hold the two loose wires of the cross-wired cable together and press the "Start" button on the Xbox. Separate the wires immediately when the green light begins to flash on the front of the Xbox. The light will begin to flash quickly and the drive will begin to spin as it enters mode "B."

6. Turn the PC on and allow it to fully boot from the DVD drive. If Windows appears to fail the boot, eject the DVD and it will finish booting. You've successfully flashed the drive.
7. Reassemble the Xbox and enjoy playing the backup of your favorite game. Sources: http://www.ehow.com/how_2316522_flash-xbox-dvd-drive.html