To view your site as it would appear on mobile or tablet, you can adjust the size of your Preview screen and Editor to mimic a smaller screen size.

When you expand the editor, you automatically narrow the preview screen.

To expand the editor, hover over the line between editor and the preview and a line with arrows on both sides will appear, click and drag to expand.

You can also shrink the size of the browser window for a similar effect. 

As you can see in the screenshot below, you can see that in a narrower screen, the navigation menu is replaced by the standard 3-line menu button on the top right of the preview screen. 

Clicking on the 3-line menu button will make the navigation menu popup vertically for easy access to each page of your website.