Use the "Magic Wand" tool to select the color you wish to change. The next step is to open the replace color box. Go to Image > Adjustments > Replace Color. After the Replace Color box is open, you may have to select the color that you want to change in the image. Sometimes, it will automatically select the color you're changing, but sometimes you will have to use the Eyedropper Tool and click a hue of the color you wish to change. After the color is selected in the "Replace Color" box, you can change the color. The "Fuzziness" slider at the top will display how much of the selection is going to be replaced by your changes in the box right below it in white. If you want the whole area replaced, move it all the way to the right. The next slider is "Hue" which will change the color of the selection/image you are changing to another shade or color. This is where experimentation begins and the preview function is helpful, so you can see your changes as they are made. The next slider is "Saturation," which will either add more of the hue to the selection or take it out. You can make it darker or lighter with this option, going from black to white and having any shade of the current hue in between the two extremes.