This is a very simple way.
Open your image
Add a new empty layer and paint with a soft brush over the hair. Be as accurate as you want.
Change the blending mode to soft light. Now you can see the hair of the woman is lighter.
Now add a Hue and Saturation adjustment layer and clip it to the underneath layer. Click colorize and change the hair in the color you like.
Easy and fast way to change the color of someones hair.
picture from pixabay.com