1. First and most difficult thing to do in changing hair color is to very accurately select the hair. In some cases Photoshop tools can be enough here. I prefer selection plug-ins. In this particular case I used Smart Mask soft mode for the job (duplicating the background layer, of course).
2. Staying in the hair selection layer go to Image > Adjustment > Selective Color.
3. Set Neutrals to -50, -50, -7, 0
4. Set Blacks to -10, -10, +5, 0
5. The result seemed a bit too red for me and I used Selective Color once again. Neutrals -50, -50, -10 and blacks -10, -10, +5, +10
6. As you can see now the girls skin looks too purple with this hair color. To adjust this go to Image > Adjustment > Variations and press More Yellow, More cyan, and Lighter with Midtones checked.
Note that these settings work good for this particular image and you may need different settings for another one. You can monitor the changes you make on the After preview in the Variations dialogue though.
The result is here:
