You seem to have developed post inflammatory hyperpigmentation
due to acne. To treat the same
Use a good sunscreen with SPF 30 even if staying indoors.
Apply Kojic acid
cream over these spots in morning and a hydroquinone
based cream in night. Use these creams in a small amount and if redness or stinging develops, stop these immediately.
Take few sessions of Glycolic acid
or TCA Peels, these would reduce the pigmentation remarkably.
Hope that answers your query. Take care.