If you want to use the histogram to change your aperture and or shutter speed to tweak an exposure you must change the settings and capture anther image. If you want to modify the image that you just captured in camera you must use the Retouch menu. See pages 174-189 in the D5000 Users Manual. You can also use Live View but there won't be a histogram displayed. See pages 41-46 in the D5000 Users Manual. Good Luck and Enjoy your Nikons!