The Curves tool in many image editors allows you to correct the exposure of your photo by tweaking the brightness and contrast. In fact, you can tweak the shadows, midtones, and highlights of a photo independently. In Photoshop Elements, choose Enhance, Adjust Color, Adjust Color Curves from the menu.
You have two ways to use Photoshop Elements' Curves tool: You can move the sliders for precise control, or you can click a style from the list to adjust the options by a preset amount. To increase the brightness of the shadows in a photo, for example, click Lighten Shadows in the list of styles, or drag the Adjust Shadows slider to the right. You can see the change in the After image in the Adjust Color Curves dialog box and also in the original photo back in Photoshop Elements' canvas.
I recommend that you make changes like this in a layer. That way you can adjust the opacity of the layer to reduce its intensity or even delete the layer entirely and get back the original photo, long after you made the original changes. Working in a layer is easy to do. Before you start editing with the Curves tool, just choose Layer, Duplicate Layer and click OK. Now you have two identical layers. When you choose Enhance, Adjust Color, Adjust Color Curves, the changes you make to the photo will be in the top layer only. For more on handling layers, see "Photo Editing Basics: Working With Layers."