Follow Steps given below to Improve an Underexposed Photo in Photoshop
(1) Open image in Photoshop
(2) Click and drag the Background layer over the New Layer icon to duplicate it.
(3) Click the blend mode up-down arrow and select Screen. The photo should appear lighter.
NOTE: The photo may look fine this way or you may need to add another layer and change it as in Steps 4 and 5.
(4) Click and drag the Background copy layer over the New Layer icon to duplicate the copy.
(5) Click the Opacity down arrow and drag the slider to the left to change the opacity of the top layer.