To modify a group training class begin in the PetExec menu under Group Training-->Modify Group Training Class.
From this screen you will see the original details when the class was added. At the bottom of the screen you will see any pets that have been added to this class roster.
Click to Modify Scheduled Dates and Times will allow you to update details for each session of the class.(See details below).
Modify Group Class times will allow you to update details for each of the sessions for a training class. In this example we have a month long class with 4 classes offered. You can then update the specific date and times for each of those classes. The number of classes listed here is determined by the "# of classes field" from the add Group Training class screen.