Hi, hope you are well. First, I need to know your height and weight. BMI does not give the whole picture. Second, chubby cheeks happen due to 2 factors high cheekbones or excess fat. High cheekbones are genetic or you can get them through aesthetic surgery. I am taking a guess that you will not like excess fat :) But still if you are underweight, I would suggest you to have energy dense foods. Have more carb based foods. Have 2 bananas every day. Fruit or dried fruit milkshakes. Handful of nuts every day. For a more detailed plan you can ping me for an appointment.