I believe this is also available in the manual of the CDJ-2000, but here you go:
- Power on your CDJ
- Press and hold the "MENU" button on the top (This enters the Utility mode)
- Use the control knob to scroll down through the list until you reach "Player No."
- Press down on the knob to select it
- Scroll through your available channels (Auto, 1, 2, 3, 4)
- Press down on the knob to select your desired channel
- Press the "MENU" button again to exit the Utility setting
Note: It may be necessary to wait a few seconds for the "Auto" selection assigns your player number before you can edit it.