Have a dermatologist or plastic surgeon take a look. First be sure that the areas are not abnormal growths that require biopsy and treatment to prevent spread or turning into something worse.
Likely there is no medical concern. If not, the hyperpigmentation spots can be from inflammation, hormones, sun exposure, age or heredity. Treatment methods include various chemical peels, liquid nitrogen, bleaching creams and various intense pulsed light/laser systems from miminally invasive to ablative.
Most important is that with any treatment, prevention of recurrence is essential, through the use of sunblock (SPF 30+ is better) and various bleaching creams on an ongoing basis to prevent post-inflammatory (after any treatment that involves controlled "wounding" of the skin) hyperpigmentation.
