How could a teenage girl lose belly fat?

Two ways. First off, get on a healthy diet. I am not going to preach on it here. Secondly, aerobic exercises. Only getting your heart rate to 120-140 BPM will be able to burn fat. The value of abdominal press exercises is unclear, but doing them won't hurt as part of your work out.
Belly fat. Belly fat is some times related to abnormal estrogen and receptors .Check lipids, Insulin resistance, thyroid lab test, and determine if she is developing metabolic syndrome, and or Insulin like factor resistance. Starting eating less fat food, low carbo, low sugar and candies, increase water , add exercises, reduce rice, increase white meat , vegetables, avoid canned food . Consult endocrinology.
See below. Monitor diet, food intake. And excercise. (some people just have problem areas, but try the above first).