Start by clicking Classes > Schedule > locate the class > click the dropdown menu and click Edit.
The first thing you'll see is a message that gives you the option to edit the Template this class series was created from or the whole series at once.
Editing the template will allow you to make the change across all classes created from that template.
Editing the series will allow you to make the change across just that class series.
Editing the individual class will just make the change to that single class.
If this last option is your goal, scroll down to the teacher and location settings. Otherwise, click the links shown in the beige message. Save changes when done.