You are 25 years old male weighing 45 kgs How do you gain weight?

Depends if you want to gain muscle or fat. If you want to gain muscle, eat things with a lot of protein, such as eggs, fish, chicken, and meat, and work out on a regular basis. If you want to gain fat, try drinking protein shakes such as ensure, and boost. They are high in calories and if you drink them regularly with your normal meals, it will boost your calorie intake and help you gain weight.