If you already took admission then you have to check if your college have mba finance or not .. then try to talk with the college dean or admission cell and tell them you are going to change then you have to find if there is any blank seat or not if not or your college doesn't allow you to do the you have to change the college is this you have to reapply on other colleges and goes through the admission process for that college for the mba finance .. 

Hope it helps you 

All the best