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 supplements .
As the area of pigmentation is large some internal medication should be taken and external treatment would be of little use. Glutathione
supplements are available as capsule and injections they can be very useful.
You need to consult a dermatologist
as dermatology being a visual field
,the treatment required can only be suggested after proper skin examination.
Some procedure can be helpful like dermabrasion