Google Classroom's homepage displays all classes of which you are a teacher or student. The classes are arranged like cards on the page. The next time you go to Google Classroom, you will first arrive at the homepage, where you can select the desired class, as shown here:

Classes in which you are enrolled as a student will have the image of the teacher on the class card.

Currently, you cannot rearrange the order in which the classes appear. Google Classroom arranges the classes in the order in which they were created.

When you are in a class, you can switch to a different class by clicking on the menu icon in the top- left corner of the banner:

The menu will appear from where you can select the desired class:

The menu is separated into the classes that you are teaching and the classes in which you are enrolled as a student. In addition, you can return to the Google Classroom homepage, view the Google Classroom calendar, access archived classes, and change the settings from this menu.