Click the menu at the top right of a lesson in the class you want to edit. Choose "Edit Class Settings" at the bottom of the menu, and then choose "Templates" at the bottom right of the box that pops up. Finally, click "Change" at the top of any day of the week, and choose the second option (e.g. "Every Friday"). Then choose your new template!
In this example, we change the Friday template for every class to a custom quiz template:
Note: When you update your template Cc only updates blank lessons that you haven't edited yet.