Classes and Class times are managed from the Planner Window.
To add a class time, select New Class on the Planner Window.
To change a class time, you need to set a finish date for the class that is currently running. You do this either by editing the class, or through the Set Last Class menu option on the date of the last class.
Then move to the date when the new class times will commence, and create a new class at that time (using the New Class menu option).