To change your layout, hover and then click (Switch Layout) on the Toolbar on the left side of your screen.
On the next screen, you'll be presented with the available Layouts and descriptions.
Once you edit a layout, a separate version of that layout will be saved, in addition to the original layout. You can either choose "Use Original Layout", which will revert all customization settings back to default, or choose "Continue Editing", which will restore all customizations you made to the theme before switching.
Keep in mind that once a theme has been reverted to the original layout there is no way to restore previous customizations.