How do i gain weight i m 23 years old n weigh 40 kgs n m6 height is inly 5 feet?

Underweight: Your bmi is 17.2 (underweight); "normal" is 18-25. Exercise (especially weight training) & healthful diet emphasizing lean proteins (& protein supplements), good fats, & low glycemic carbs (fruits/veggies/whole grains), low-fat dairy should add some muscle weight. Consider joining a health club where trainers can guide u. Have doc check thyroid function & blood sugar.

Eat healthy foods: high in calories. Eat 6 meals /day. Work on strength training to build large muscle groups. Work out w heavier weights w fewer repetitions. Sleep at least 8- 9 hrs/night. Hydrate well w water. Foods to help w weight gain: almonds, cashews, pecans, sunflower seeds, flax seeds, walnuts, hummus, avocados, granola, trail mix, dried fruits, protein shakes & bars, lean meats, poultry, fish & smoothies.