Concrete CMS separates the visual representation of a page with its functionality through the use of page templates (visual) and page types (functionality).
If your website is set up with different page templates and/or themes for the page type you are editing, then changing the visual representation of a page is easy in concrete, just click on the Page Properties icon and select the Design section: 
If you want to change the Page Type, find the page in the Sitemap, click on it and choose "Design & Type" and change the Page Type from the "Page Type" select field and click "Save".