I am 22 years old. height is 165 cms. but my weight is only 40kgs. how to increase my weight?

Healthy food...: Your BMI is only 14.7 - this is underweight. See doctor to ensure no underlying medical problems - Learn healthy habits - nutrient rich whole foods, small amounts of healthy fats (avocado, coconut oil), small regular meals (5-6 per day), regular exercise (cardiovascular and strength training) - is what is best for you. I would recommend against artificial/chemical supplements to gain weight