To change a pages template move your mouse up to the upper right and mouse over the palette icon, this is the template menu. This menu will have three options. One of them is "Change Template". Click on this and a small window will popup. This window will have a dropdown of all available templates for this page. Choose one and click the change button. If you want the template you choose to be the default template for all new pages on this site then check the checkbox that says "Set this template as the site default".