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 Products>Classes and then on Schedule.

2. After you click on the activity, scroll down to the Attendees 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 Edit booking.

Done, your client will be informed of the changes.