Many editing programs allow you to edit the Brightness and Contrast of an image.  Adobe Photo Shop has a couple of more sophisticated ways of improving the look of an image.  This page discusses the Levels tool

 Within Photoshop

1.      Open your image,

2.      select Image\Adjustments\Levels

3.      A histogram of the image is displayed with some controls….
RGB (to change all three color channels at once)

o       Input Levels
Black Point - moving to the right shifts the lowest “black” value so any value beyond this is set to “black” This makes the image darker, but loses detail in the Shadows.
Mid (grey) Point - moving to the right makes the image mid (grey) point darker, moving to the left makes the image mid (grey) point lighter
White Point - moving to the left shifts the highest “white” value so any value beyond this is set to “white”.  This makes the image lighter, but loses detail in the highlights

If there is a section of the image you know should be absolute white (or black), you can use the appropriate eye dropper tool to set the Min/Max white/Black point and Photo Shop will adjust everything else in the image to fit along that new “tonal start point”.

o       Output Levels
In-lab, we rarely use this adjustment and the allow further adjustment to contrast.

4                    This control (above) can be exerted over each color channel separately if desired and can be another way of doing selective color adjustments to the image