Installing a New Joomla! Template and Setting It As Default
From your /administrator link, go to Extensions->Install/Uninstall
In the Upload Package File area, click the Browse button to locate your new template on your local computer. Then, click the Upload File & Install button.
Now, browse to Extensions->Template Manager, and you should see your newly added template in the list. Check the tick box next to it, and click the Default button at the top right to set this new template to display on all pages (or rather all pages to which other templates are not specifically assigned)
If for some reason your server will not allow you to upload templates through the installer, you can simply unzip the template on your local machine somewhere and then ftp it up to your server into the /templates directory.