Depends . Some people just have dark skin in that area of the body. That cannot be changed. Others have a condition called acanthosis nigricans. Weight loss helps with this.
Nothing. There's no real treatment for the darkening of the skin that can appear in these areas. Sometimes if women are overweight or obese, weight loss can help as the skin darkening is due to imbalance of hormones typically associated with pcos. Otherwise, the only real method would be skin bleaching, but i wouldn't recommend that at all.