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 actually default template for the Front-end is marked with a star, in the image below it is rhuk_milkyway.
Change the default template to beez:
the star changed from rhuk_milkyway to beez and you are done.
Assign more than one template for the Front-end
Joomla! gives you the possibility to switch between two ore more templates on Front-end. This will be done with "Menu Assignment", so you can set different templates for each menu or only a menu item.
Click on: Extensions -> Template Manager
You see your installed templates, now choose a template which is not set as your default one. Click on the template name or hit the radio button left from template name and click "Edit" on the toolbar. You now see the page "Template: [ Edit ]", click the radio button "Select from List" in the "Menu Assignment" block and choose the menu items on which the template will be shown. Click "Save" on the toolbar and you´re done.