If you don't want to cancel a booking and then create a new one, you can just edit the current one as long as it is the same activity.

1. Click on Calendar, and click on the class in which your client is currently booked

2. After you click on the class, scroll down to the Booked section. Click on the 3 dots of the booking you want to edit and then Booking details.

3. On the Date and time section, choose the new schedule for that booking and click on Save.

Done, your client will be informed of the changes.