Curves is an extraordinary adjustment command. It allows you to do all of the adjustments in Levels plus more specific adjustments. To access Curves, go to the Image menu and choose Adjust. Choose the fourth option "Curves." As it opens a graph appears that is very different from the Levels histogram. A diagonal line cuts through the center of the graph. Click somewhere around the center of the diagonal and a dot appears on a point on the graph. Holding down the mouse button on this dot drag the dot and watch how the image changes. Click on another point and drag the mouse again. When you are happy with what you have click the Save button.