How to brighten underexposed photos in 4 easy steps

1. Open an Image

Open an image in Photoshop File --> Open

 2. Duplicate the image layer

Select the image in Layer tab and drag it to create a new layer icon at bottom of layer tab or 

Layer --> New --> Layer via copy (ctrl+j)

3. Change the Blending mode

Select the duplicated layer, Then change the blending mode to Screen

4. For more Exposure

For more brighten exposure duplicate the layer again, If it exposed more then less in the Opacity of the Duplicated Layer

Final Result: