How to gain a body weight upto 10kg?

Weight gain: I believe you are asking the best manner to gain 10kg in body weight. I will presume you to desire to do so in a healthy manner adding desirable muscle rather than fat. This would be best achieved by stimulating your body to do this naturally by lifting weights hard through strength training while eating in a healthy manner -ideally grass fed whey protein or good quality grass fed beef.

See Dr., then eat...: If one is truly underweight (based on height, family background, and body type), a Dr. can evaluate for a hormonal, digestive, or other disorder. If no medical cause is found, then one can add weight by eating more calories of nutritious foods, while also doing weight-training to increase muscle. Proteins: egg white, chicken breast, salmon,... Better fats: olive oil, peanuts, almonds, tofu,....