I'm 23 years old and my weight is 40Kg.how to increase my weight?

Protein shakes made for weight gaining. They taste good and they will help. Drink one before bed at night. Exercise and eat healthy but make sure you eat a lot. If you like pasta, try using olive oil to flavor it. If you are a girl, eat more soy products. It will raise your estrogen and that will increase your appetite and make you gain more weight.
Good luck