The first thing you should do is talk to a doctor. But if you cant for some reason then👇
Depends on her height, but go to google and look up BMI it should lead you to a website that will ask her age height and weight (sometimes it asks for parents height too) just fill all those in and it should tell you if shes at the correct weight. And if she is over weight don't talk to her too much about it cause it could affect her in a bad way. Just have healthier food in the house, unless its serious and putting her in danger, then defiantly ask her doctor for advice.