You should consult a dermatologist and get prescribed various skin creams like hydroquinone, azelaic acid, corticosteroids, tretinoin cream and
laser treatment. If it still persists then you can go in
for laser treatment, Chemical peeling or microdermabrasion. Also then the possibility of acanthosis nigricans has to be ruled out.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided.