Swap the teacher for one session, all future sessions, or the whole series — with automatic notify.
Click the session on the Schedule. The dialog has an Instructor picker.
Choose a different teacher and select the scope — single occurrence, this and all future, or all in the series.
Klasly automatically emails confirmed-booking members about the swap (e.g. "Sarah → Mei") so they're never surprised on arrival. Untick the "Email confirmed members" box if you're doing a silent data fix.