You can make changes to your reservations, generally up to 4 hours before the session starts (please check How much in advance do I need to cancel a class to avoid losing any credits? for more details).
To change the teacher of your 1-on-1 sessions, you can do the following:
From the web version:
Log into your account..
Go to My Schedule → Booked Sessions.
Click on the class that you’d like to change.
Click on Edit → Change Teacher.
From the APP:
Log in with the APP.
Go to the Schedule tab (4th tab at the bottom).
Go to Booked.
Click on the session that you’d like to update.
Click on the pencil icon in the top right corner.
Choose Select a new teacher from the drop-down menu.
Pick another available teacher.
Confirm your changes.