How do you change the midtone values of a photo?

Here's the short answer: Start a Levels Adjustment Layer and move the midpoint (gray triangle) slider to the right or left. Moving it to the right will darken the midtones. Moving it to the left will lighten the midtones. Either way, be sure not to move the black triangle or the white triangle!

And here's the explanation: We use Adjustment Layers because we don't want to harm the pixels in the Background Layer by repeatedly changing them. Adjustment Layers allow us to "suggest" changes by visually changing the image without actually making a change until we're sure of what we're doing. This is called non-destructive editing.

Start an Adjustment Layer in your Layers panel by clicking on the half-circle icon at the bottom of the Layers window. Choose Levels. Move the gray midpoint slider to the right or left until you like the image you see. Click OK, and you'll see your Adjustment Layer above the Background layer. To make additional changes, double-click the icon for that Adjustment Layer. The Levels panel will open again for you.

Don't forget that if your photo has layers above the Background layer, you must save it as a Photoshop (.psd) document to maintain those layers!