This depends on whether the payment has been made via Stripe (legacy payment option) or Crezco (bank transfer). If you're not sure you can see this on the invoice associated with the booking.
Crezco (bank transfer):
At present there is not an automated refund feature available though this is pending as the new open banking processes become more developed. As such you will have the option to rebook or liaise the with the client directly to do a bank transfer of the funds back to them.
Stripe:
If it is more than 24 hours before the booking then you can cancel the appointment and the refund will be processed automatically. If it is within the 24 hours then due to the cancellation rules then the money is already on it's way to you thus you will need to either rebook the person or manually process a refund.