When editing a specific page - you can change the page layout in a couple of places, using the grey tabs at the top of the editor. LAYOUT best tab to choose, but if you're already on the HELP tab, changing the layout is one of the options available as well
ABOVE - A standard editor window with the HELP and LAYOUT tabs indicated.
There will be a list of related page styles available. If a page layout isnt available, consider adding a new page (ADD PAGE button on the left hand side of the CMS )
ABOVE - Example page layout types available. Available page types change depending on the existing page type.
The layout for the current page is indicated in the tooltips bar on the right hand side of the editor