You can re-arrange music in the playlist by simple selecting the items and dragging them to a new position in the playlist. Even if you drag an item which is currently playing, CDJ will continue to play that item.
You can also delete any playlist items by selecting them and then hitting either the delete key or the delete icon in the playlist toolbar. To delete all items from a playlist, you can press the delete playlist icon on the playlist toolbar. If you delete an item which is currently playing, CDJ will start playing the next valid item in the playlist.