The best way to gain lean muscle mass is to lift heavy things. Following that plan should increase your muscle mass. Of course reducing your body fat percentage is the best way to get sculpted. We all have decent muscle mass on our bodies but it is often covered with fat. Reduce the fat and you'll look more muscular.
Since maintaining muscle takes a lot of energy your body won't do it unless it senses that it is necessary and it is provided the resources. That's why you have to lift heavy two to three times a week and provide it with proper nutrition.