Thank you for your answer!
I would check if it is a real sunburn, or if it has been there since birth which might be a facial angioma, or a nevus. It is due to post inflammatory 
hyperpigmentation. Anyway there are a lot of procedures you can do in a 
dermatologist office, from 
chemical peels (glycolic, TCA), to fractional laser resurfacing, microneedling and 
PRP injections.
First i would suggests to my patient start with a bleaching agent at bedtime, such Mela Cream Dermaceutic, or Yellow cream Dermaceutic.
There is also a prescribtion drug 
Tri Luma, so you can talk with your treating physician. I let my patient use this product at home, always protecting the skin from the sun with a product with SPF 20 or higher in order to avoid further hyperpigmentation. After a month the patients starts the in office procedures depending on the age and skin phototype.
I hope this helps.
Feel free to ask me again.