You have 2 ways to install a free WordPress theme. The first thing that you have to do is to download the theme to your PC. Once it is done, you can install it using:
The standard WordPress "Add New" theme option in the Appearance -> Themes. Click "Upload Theme" button, choose the zip file that you've downloaded from our website and click "Install Now". Once it's done, simply activate the installed theme.
Please connect to your website server via FTP. Unzip the downloaded file, and use the FTP software to upload the files into a web host directory of your choice. If you want to install WordPress on your root domain name (like example.com), then you need to upload WordPress files to your website's root directory. Usually this directory is called /public_html/wp-content/themes. Once it's done, please go to Appearance -> Themes and activate the theme.