How can i put on another 7 kgs in 3 or 4 months?

im 14 don't have Access to weights Only 2.5 and 3 kg ones i am 5 foot 7 and 50 kgs and on the bmi thing it said i am underweight so i am going to try putting on some weight i only have to put on 5 but i want to put on 7 so i have a tiny bit of bulk