With every reservation you make, there is the option to cancel it.
Log in to your dashboard and go to Bookings > Bookings.
You can find the reservation in your booking list and click on the 'trash bin' to cancel the reservation. Please confirm the pop-up to remove the reservation finally.
Note:
This functionality is possible if the reservation isn't converted into a booking yet.
After the cancellation, you will get a notification, and the tickets will be available in the shop again. There will be no refund needed, since it was a reservation.
Note:
A reservation remains till the reservation changes into a booking or is canceled. The reservation won't expire automatically.