When you edit a class, choose whether to push the change to sessions already on the schedule.
Change a class's capacity, price, instructor, room, online link, visibility, duration, or type and save.
If a session-relevant field changed (instructor, room, price, capacity, …) and the class already has sessions on the schedule, a prompt shows how many. Choose "Update the N upcoming sessions too" to push the change to those future (non-cancelled) sessions, or "Only future new sessions" to leave them as they are — only sessions added later get the change. Changing the instructor and choosing "Only future new sessions" leaves the existing sessions with the old instructor.