This is a little more complex. This is better if you have an image with bright highlights and want to bring up the shadows in exchange for flattening the highlights a little. (see also Brightening Dark Shadows.) Deliberately underexposing a digital camera in contrasty light and then doing this actually improves the image over a conventional exposure, because it gives you the ability to create a shoulder for the highlights that's missing in digital cameras.

Call up curves by IMAGE > ADJUSTMENTS > CURVES or simply command + M.

Put the cursor on the lower middle of the diagonal line. Click and hold it and drag it up. Have fun, you can do an awful lot here.