Belly fat

Patient: So im a 15 year old female, i currently weight 51.3 kg but its constanlt changing between 49kg to 53kg.My height is 5’5When I try to calculate the weight to height ratio it keeps on saying in underweight.But i have a lot of belly fat that i want to get rid of.The rest of my body is thin but the belly fatDoes anyone have an idea how to deal? because i would really like to get rid of all the belly fat.