To change your default Joomla template with one of the installed ones you have to:
- Log into your Joomla Administrative Panel.
- Click the Extensions tab.
- Select the Template Manager.
- Put a tick on the template you like and hit the Make Default button.
*Note: on some of the templates under Location it says Administrator. They are for the Administrative Panel and for the front-end you have to make sure that you select a template with the Site location.
More details on how to install additional templates to your Joomla can be found here.