You have multiple dark patches
on neck, arm and trunk...looks like little burn
These might be..
i) Lichen planus pigmentosus
ii) Fixed drug eruption
iii) tinea Versicolor
iv) Para psoriasis or
v) Tinea corporis
But rare chance of being an acanthosis Nigricans
It would be a wise decision to confirm your diagnosis first then manage it.
You can consult a dermatologist and have a Wood's Lamp study, skin scraping for fungus and skin biopsy for histopathology.
Youu diagnosis and management will depends on that report.