I’d look into a variety of things such as: selecting the hair and creating a new layer, filling it with the brunette color you’re after, changing blending modes, masking areas if needed, dodging, burning, and possibly adding additional duplicate layers until you achieve the desired look. Adjust levels, curves, whatever until you get what you’re after. You’ll probably find it’ll be a combination of tools/filters/etc to get what you’re after and to make it look believable. One thing you can consider is to bring in a target image of the brunette hair look/color you’re after, so, as you’re editing, you have a reference to go by.
https://www.graphicsdump.com