Head to Classes >> Reservations.
If a reservation is still editable you will see a drop-down option besides it. It won't be editable if it has completed, for example.
Note. You can also delete the reservation, but only if it has not yet begun. Deleting will completely remove the reservation from the system.
You have a couple of options when editing a recurring reservation.
Start date - edit the date the reservation begins.
Duration - Change when the reservation ends. It may be that you want to end it immediately if for example, a customer leaves your business.