Click on the "Image" menu at the top of your screen.
Select "Adjustments" from the drop-down menu.
Choose "Replace Color…" from the Adjustments options.
A "Replace Color" dialog box will appear.
Click on the "Sampled Colors" option in the dialog box.
Use the "Eyedropper" tool (the icon looks like a little dropper) that appeared in the "Replace Color" dialog box to click on the color in the image that you want to change. This adds the color you want to replace to the selection.
To add more shades of the selected color, you can use the "Add to Sample" tool (the eyedropper with a plus sign).
After you've selected the color you want to replace, you can use the "Hue," "Saturation," and "Lightness" sliders to change the selected color to the new color you want.
Once you've made the color changes you desire, click OK to close the dialog box.
I was initially attempting to do this on a largly black and white photo. I later read that the replacement tool doesn't work well with black and white so I attempted to do it on a picture of trees. Still no result.
I should mention that I am a complete newbie to this so it is likely I am missing a step somewhere...