I'm 19yr old girl wid pcos my insulin levels r proper, periods r regular but I hv dark neck, under breasts n bikini line how do I get rid of dark region?

Difficult. You can see a dermatologist for evaluation and treatment. There are medications that can lighten darker areas of the skin with some degree or success. Good luck.
Lower your insulin. You likely have a skin condition called acanthosis nigricans which is caused by Insulin resistance and is common in women with pcos and diabetes. Regardless of your Insulin levels on lab tests, it is still the most likely cause. You need to cut back on carbs, and if overweight lose weight. If you do not see improvement you need to see a hormone or skin doctor.