Once a booking has been processed and approved, you will need to contact the service provider / business you made the booking with to cancel the booking.
Each provider / business will have their own cancellation and refund policy so any refunds are at their sole discretion.
Communicate with the provider / business directly to arrange a new date/time, cancel your booking or request a refund (contact details can be found in your booking confirmation email and on their Hoofpick profile).
You can check out how to change your booking for a different day or time here.