A ticket that was issued as part of an order is eligible to be voided (if the purchase price was zero), or refunded (if the purchase price is greater than zero).
Tickets not issued through an order can be invalidated or deleted. See relevant article about the differences of those two options.
Locate the ticket you'd like to void or refund
From the dropdown button next to "Edit" select either Void or Refund.
Repeat process for any other tickets belonging to the same guest.
A new slot will automatically open up for the event after refunding or voiding the ticket.