Several ways to do this.
A method:
select the hair, go to image > adjustments >levels. Play with the gray and black arrows till you get what you want.
B method: Use curves
image > adjustments > curves. You can play with all chanels or RGB separately. Same with CMYK.
C method: Use Color balance.
Image > Adjustments> color balance
Using color balance you can play with darks, midtones and highlights separately.