To change your layout, hover and then click (Switch Layout) on the left-hand toolbar.
On the next screen, you'll be presented with the available Layouts and descriptions. Click "Use this layout" to try out a new one!
Once you edit a layout, a separate version of that layout will be saved, in addition to your original layout.
If you decide you want to go back to a layout you have already made edits on, your original layout for example, you can either choose "Continue Editing", which will display all customizations you made to the theme before switching, or choose "Restore Default Layout", which will revert all customization settings back to the default.
Keep in mind that once a theme has been reverted to the original layout there is no way to restore previous customizations, so use "Restore Default Layout" cautiously.