Hi.. Diet changes are needed to increase body weight. The calorie consumption should be higher than what is needed for your physical activity.. In general increase the protein consumption either through diet changes or through protein supplements.. Or alternatively take a diet plan from any nutritionalist. They will be able to calculate your  calorie requirement as needed and make necessary adjustments and help you put weight...

you may require a diet chart which should have a diet rich in calories and carbohydrates,fats etc. While not being much heavy on metabolism too,so work gradualy and steadily.