If there isn't anything like a mole to treat, and just changes in the pigmentation, there isn't any way to treat this, unfortunately - https://www.webmd.com/parenting/baby/birthmarks#1
There are some other things you can do, though - some use cosmetics to darken it, others use medical tattoos - https://www.realself.com/question/melbourne-fl-darken-birthmark Talk to a licensed plastic surgeon about that.