On the Dashboard, in the upper right corner, click on "Manage classes".
Under "Manage classes" click on "New class".
A new window will open up where you will be able to choose if you want to copy any of the content from the current class that you are in. You can choose between users, groups, course materials, tests and settings (it's not mandatory and you can also decide to create a blank new class).
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Give it a name and click on "Create class".
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The class will be created, and you will immediately be placed in it. You can switch between your classes as usual, in the drop-down menu.