1. Open overexposed images: over.jpg, over2.jpg and over3.jpg
2. Duplicate background (Layer - Duplicate Layer...) and change Normal (blending mode) to Multiply
3. In order to selectively remove this darkening effect, click on Add layer mask button and use soft, black-color brush to remove applied effect from some parts of the image.  If you remove too much, to undo, paint with white-color brush.  To cover big areas of image, try using black and white Gradient Tool instead of brush or combination of both..
4. Repeat duplicating layers if needed, and fine tune by adjusting opacity

Multiply
(to darken)

1. Open underexposed images: under.jpg, under2.jpg and under3.jpg from exposure folder
2. Duplicate background (Layer - Duplicate Layer...) and change Normal (blending mode) to Screen
3. In order to selectively remove this darkening effect, click on Add layer mask button and use soft, black-color brush to remove applied effect from some parts of the image.  If you remove too much, to undo, paint with white-color brush.  To cover big areas of image, try using black and white Gradient Tool instead of brush or combination of both..
4. Repeat duplicating layers if needed, and fine tune by adjusting opacity

Screen
(to lighten)

You can intensify the colors of a dull photograph quite easily in Photoshop. What you may not know is that pumping the saturation (using the Hue/ Saturation dialog box) isn't the solution. When used indiscriminately, the Hue/Saturation controls can actually distort neutral tones.

Instead, use Photoshop's layers and blending modes to intensify colors while preserving the relative tonal values of a photo. Open the image file and make a duplicate layer of the background. In the Layer palette, change the blend mode for the new layer from Normal to Overlay.  Try also changing the blending mode to Soft Light. (If you are looking for extreme sharpening, use Hard Light instead.)

1. Open overexposed (too light) image: e0.jpg
2. Duplicate background (Layer - Duplicate Layer...) and change Normal (blending mode) to Overlay (note increase in contrast)   or
   Duplicate background (Layer - Duplicate Layer...) and change Normal (blending mode) to Soft Light
3. Lock and unlock layer to examine the result.

You can control the strength of the effect two ways: You can reduce the effect by increasing the transparency of the Overlay layer (using the opacity slider in the Layers palette). Or you can intensify the effect by adding more copies of the Overlay layer. But this method increases the contrast.

Overlay
(increase in contrast)

Duplicate

Soft Light