After starting the event, the slide-deck you added to the class template will be available in the classroom environment. To present the slides, make sure the "Present Mode" toggle is on and use the left navigation panel to access the "Presentation" section.
Here, you can move between slides, see the notes you provided for each of them, and see a preview of the next slide.
If at any point in time you wish to switch a slide-deck, simply edit the class template, and apply the changes. This will automatically replace the slide-deck for you and your students.