There are 2 ways to install drupoal themes one is automatically from the admin interface.

Automattic Installation

Log in as site administrator go to the theme manager by selecting Appearance and here you will find two option to install your theme

1. Install from a URL

In this option copy the path of the theme zip or tar folder (something like this http://ftp.drupal.org/files/projects/name.tar.gz) and click intall =

2. Upload a module or theme archive to install

Manual installation

For manual installation you requires to have the ability to move files from your local machine onto your server and this should be done either by FTP client or through web hosting control panel.

1. Download the .tar.gz file from Drupal.org
2. uncompress the archive file
3. Once successfully uncompress, take a look inside the directory. If the theme directory includes a README file, read it, and ,make sure you have not missed any system requirements or terms of use for the theme.
4. Using FTP or Control panel navigate to to your drupal installation folder on server then navigate to the directory sites/all, this is where you should place all third-party themes and modules now upload the extracted theme files here.

Using the sites/all/themes directory, instead of the themes directory, allows you to run multiple sites on a single Drupal installation.

Finally access the admin interface for your site on browser and go to the theme manager. Now You should see your new theme listed on the page

Thanks