What is the best way to treat dark spots on skin?

Depends on what . they are. These lesions would need to be examined, a history obtained & other tests may be necessary in order to diagnose them. Usually the lesions needs to be identified to determine the solution.
See Dermatologist. You need to have the dark spots evaluated to determine what they are, then they can be treated properly. Most dark spots are ok but should never be taken for granted. See your skin doctor.