Out of all the remarkable things you can pull off in Photoshop, changing someone's hair might be my favorite thing to do. Besides being just plain fun, it'll exercise a lot of techniques that you'll want to have mastered in many other situations as well. Check out this great tutorial on swapping hair.
I used to like to change my hairstyle a lot when I was a kid, but the problem was always waiting for it to grow out enough to try a new one. If I had been better at Photoshop then, I wouldn't have had to wait. Coming to you from Jesus Ramirez of the Photoshop Training Channel, this tutorial will show you how to plop an entirely different hairstyle on top of someone's head. In it, you'll learn how to select a proper candidate for replacement based on matching lighting, followed by how to make and refine selections relatively quickly, masking, color correction, and distorting the hair to fit the head while making final color and levels adjustments. It's not an overly tricky process, but it does exercise a lot of your Photoshop muscles and is worth taking 15 minutes to try out, even if you don't plan to use the techniques for this specific task.