Your approach will be different, depending on whether you are just changing instructors, deleting one class time, or not offering a particular class anymore. Here are your options:
Delete or Cancel One Class Incidence
If you want to delete or cancel an upcoming incidence of a class, you can do that right from the calendar view...
Or you can go into the individual class time to delete it by clicking on the Actions dropdown and then clicking Delete Class:
If you want to delete a class from the schedule you'll need to delete any reservations first, it's possible to cancel a class and leave the existing reservations intact.
Delete a Recurring Class Time
To remove a recurring class from your schedule (effective immediately or from a date in the future), go to Manage Classes, select the class, and click on "Manage The Repeating Schedule".
From there, you can delete the recurring class or select Edit to set that up for a date in the future.
Delete or Archive a Class Type
If you actually don't offer a particular class type anymore, you can archive it via Manage Classes.
From there, you can select the class description you no longer offer & "Edit the Class"
Archive it here and so it doesn't appear in your active class views anymore. The history for the class will still exist for reporting purposes. You can only delete a class type that has no history.