What diet and workouts to do, to gain muscle if currently underweight?

Diet + weight liftin: In men and women, resistance training combined with a healthy diet will build muscles....More in men than women but it works for both. Stronger muscles tend to burn more calories than weaker muscles even when you are not using them...That helps reduce fat. Lifting with proper form to the point of momentary failure plus a good diet seems to be the best way to get the muscles to respond.

Dr Ali and another doctor agreed

Eat , exercise more: To gain weight, you prefer to gain muscle than fat. You still need to take in more calories than you need, then you need to work the large muscles (thighs) , of the body, by strength training. It takes time and effort and consistency and persistance. In general, body composition, doesnt change more than 1% in three months. So it could take up to three months to even see a 1% change in muscle mass.