If your current project is using Normal Template, you can change Page Layout in WYSIWYG editor of each page by using "Layout" tool in the toolbar.
If your project is using Responsive Template, you will be able to select page layout only once you create the page. It doesn't allow you to change the page layout later. This is to avoid webpage broke when open in mobile device solution.
If you want to use a new page layout, you'll need to create a new page , select page layout you want, and insert the content again.