Installation Package
You can install a new template to your Joomla 3 from the admin area > Extensions -> Install/Uninstall.
Installation New Joomla! Template
To install a new template in Joomla! 3 CMS login to your Joomla website as administrator, by simply entering the url_of_your_website/administrator in your Browser. Enter the appropriate username and password {which was provided while installing Joomla} and then click on Log in.
After logging in as Joomla Admin, a Control Panel page in the top menu, locate Extensions. Click on Extensions, from the drop down menu that appears click on Extension Manager.
An Extension Manager page opens. In the Upload Package File section, click the Browse.
After clicking on the Browse a file navigator window opens. Navigate to the appropriate folder where the Joomla template file is located. Select and open the Joomla Template zip file.
The zip file name appears in the Package File text box. Click Upload & Install in order to upload the Joomla template zip file and install it into the Joomla.
Once the template zip file has been uploaded you will see the successful installation message.
In the top menu, locate the link Extensions. Click on Extensions, from the drop down menu that appears click on Template Manager.
A list of templates in Template Manager is displayed. The template which is active has a yellow color star icon under the Default column, in front of the template.
NOTE: You can assign template both to the Site (Frontend) as well as Administrator (Backend).
To make the template currently installed the default template, select the checkbox adjacent to new template name and then click the Make Default icon at the top-right hand corner.