Lightening an Underexposed Image, Method 2
Another way to lighten an image uses the Overlay screen mode. Here's how: Open the image and click on the Create a New Layer icon at the bottom of the Layers panel. Press D to set the Foreground/Background colors to their default black/white, respectively, and then press Command-Delete (PC: Ctrl-Backspace) to fill the new layer with solid white. Change this layer's blend mode to Overlay and you can then adjust the brightness of the image by changing the layer's Opacity until it looks right to you.