You'll probably have to reschedule, either by calling or in person. They might try to get you in sooner if you explain the situation (if there are any extenuating circumstances). Sometimes there is a fee, you'd have to check out their policies to find out. If there is, they'll probably send the bill to whatever address they have for you. If they have an online portal thing, you might be able to check and maybe even pay there.
the circumstances are my car started breakong down and in my panic to arrange an appointment to deal with that I forgot the appointment that I already had. thank you