$1700.00 to change flight

My daughter booked a flight through Jetstar and as I cannot fly on those dates due to work commitments I asked to change flights and yes Jetstar can do this at a cost of $1700.00 which I simply cannot afford my daughter who booked is only fifteen and she got excited about the cheap 109.00 fares to bali is there someone at Jetstar who I can contact . i thought you had twenty four hours to change flights?

International
Australia-Sydney
Indonesia-Denpasar (Bali)