Sometime reducing weight by working out is easy but doing the opposite to increase your weight is really challenging.
To increase body weight what you have to do is, eat more but eat healthily. When you go on eating fast food, the the body will increase but not it will not be healthy. So eat only healthy food.
Eat food which contains more calories. I suggest you to have a proper diet plan which make you healthy and will increase your body weight. Though your body increases you should look healthy not Fat.
So have proper diet, eat food/ fruits with rich vitamins.