Underweight is a term describing a person whose body weight is considered too low to be healthy. The definition usually refers to people with a body mass index (bmi) of under 18.5 a weight 15% to 20% below that normal for their age and height group.
Low weight may be due to improper diet, reduced assimilation from intestine, illness, metabolism, genetics etc.
Thus first we have to rule out any such condition, with this have a healthy diet with more proteins (legumes, eggs, fish, lean meat, vegetables and fruits, drink plenty of water and go for regular exercise.
A homoeopathic constitutional treatment will definitely help you a lot.