Thanks for writing in.
You seem to have postinflammatory hyperpigmentation
For pigmentation as a general rule-
Avoid sun exposure
Use a good sunscreen of SPF 30 and repeat application every 3 to 4 hours
Take antioxidant and vitamin c
You can apply cosglo cream at night time over the affected area.
You need to consult a dermatologist
as dermatology being a visual field
, the cause for your pigmentation and the treatment required can only be suggested after proper skin examination.