How can a teenage girl lose belly fat and a bloated rounded looking stomach?

Fat? Is it really fat? Genetics determine where you store it most (belly or hips/thighs). Excess calories get stored as fat. Rounded/bloated look, or really bloated? Bowel habit? Muscle tone? Diet intake? Iron status? Alcohol intake? Physical activity? Menses regular? Adequate protein intake? Seen a doc to get examined? If not, do so. All of the above factors may play a role. Others might as well.
Appointment. I would want to see this teenage girl and evaluate her. I want to be sure it is a clinical issue and not a body perception issue. Some people have round "bloated" bellies from poor digestion so a diet history is #1 on the list. If she is eating too many carbs and not digesting them but "fermenting" them she will have a distended looking belly. From there we create the best plan for her.
Bowel habits. Have to be sure she is going to the bathroom , need to have three bowel movements a day, the sludge will definitely add up to the belly and put on weights. 5 pounds weight gain for not going in one day! sum it up.Drink plenty of fluids too , decrease carbs, more proteins and fibers.