How do I get rid of darkening skin on some parts of body?

Many options. Excess pigmentation or hyperpigmentation can be caused by hormones, pregnancy, sun damage, friction, rubbing, injuries or trauma, medications, rash, acne, or procedures. Treatment is with topical bleaching creams, retinoids, glycolic acid, chemical acid peels, microdermabrasion, lasers, ipl.
Dark skin. Hyperpigmentation is defined as excess pigmentation in a bodily part or tissue, including the skin. Hyperpigmentation removal treatments target darkened areas of your skin, which may be a result of hormones, sun damage, injuries, your skin type, medications or acne. Treatment is with topical hydroquinone, retin a (tretinoin) cream, chemical peels, laser or microdermabrasion.