With every reservation you make, there is the option to cancel it.
Login into your dashboard and go to Bookings>Bookings.
You can find the reservation in your booking list, click on the 'trash bin' to cancel the reservation. Please confirm the popup 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 an notification and the tickets are available in the shop again. There will be no refund needed, since it was an reservation.
Note
A reservation remains till the reservation will change into a booking or will cancel. The reservation won't expire automatically.