Using software packages like Adobe Photoshop the changing of contrast is a simple task. There are a number of ways to do this and the simplest of these is by selecting Image-adjust-Brightness/Contrast from the menu bar as we show below.

Moving the sliders within this palette will increase brightness and contrast to your taste. A better and more controllable way is via the levels commands which is found by selecting Image-Adjust-Levels or the shortcut key Ctrl+L. (see below)

By moving the sliders on each end of the histogram you can set the levels of the highlights and shadows.

You will need to experiment with the levels command, but by manipulating the imput and output sliders you will see the contrast change. With the example shown above I have digitally simulated a darkroom test print showing the equivalent of printing grades of about 1-4. By making a selection of a part of your picture you can alter the contrast of just selected parts which allows enormous control over the end result.