To change the default Template for the Site (Front-end) or Administrator (Back-end), follow these steps:
You will now see the Template Manager screen.
Note: If you do not see Template Manager listed as an option on the Extensions menu, then it is most likely because you are not logged in as a Super Administrator. Only Super Administrators will see this menu item.
From here you manage your templates for Front-end and Back-end. The default view shows your installed Front-end templates. The default template for the Front-end is marked with a star; in the image below it is protostar.
To change the default template to beez3:
the star changes from protostar to beez3 and you are done. Alternatively, you can click on the shadow of a template's default start to quickly set the template as default.
Joomla! gives you the option of switching between two or more templates on the Front-end. This is done with "Menu Assignment", so you can set different templates for each menu or only a menu item. This allows specific "pages" that are defined by menu links to set for a "specific" template.
Click on: Extensions → Template Manager