Click on the "Menu" button at the top.
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The new button will be added to the menu and the navigation panel at the left panel.
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Change the name of the submenu by clicking the text in the menu.

You can customize the submenu by navigating to it. Navigate to it by hovering over the submenu on the menu page and clicking the arrow that appears above it (seen below). Or, you can navigate to it by clicking the submenu in the navigation panel on the left.