Hello there...I can understand your concern. Ideal weight for your height is around 73 kgs. You are not too underweight. If it is just your arms you are worried about, then you can start doing excercises like push-up, lighting weights. Work the biceps, triceps and the muscles of forearms. It will help build the muscle mass. Eat foods rich in proteins like pulses, soyabeans, vegetables, fruits, egg white, boiled chicken.