How can one lighten up dark scars on skin?

Depends upon the age: If this is a new injury - i would suggest waiting a period of 1 year for the scar to "mature." if the scar is older, there are a variety of prescription medications called "hydroquinones" that can be used under controlled supervision.

Avoid sun & bleach: Avoiding sun exposure and inflammation as well as the use of peeling/bleaching agents commonly containing a combination of steroids, retinoids, and hydroquinone are the most commonly utilized treatments. On o casino chemical peels, resurfacing, laser treatments, ipl and fractionated therapy may be also used.