You don't reapply. First, you need to call the admissions office and speak with a live person about whether or not you can now accept their offer of admission for fall 2019. As others noted, universities generally fill their enrollment by offering open spots to other students on waitlists. Each university is different so you have to speak with them about their process and if there is still a chance for you to attend.
If that is possible, you do what you need to do to accept, pay your fees, get housing, etc. You also contact the university to which you will no longer be going and they them know. Again, depending of the timing and the school, you may or may not be on the hook for some fees. You generally won't be liable for tuition if you haven't actually registered for classes.