Unzip this file in your site's packages/ directory.
Login to your site as an administrator.
Find the "Extend concrete5" page in your dashboard.
Find this package in the list of packages awaiting installation.
Click the "install" button.
(Optional) Under "Swap Site Contents", select "Yes. Reset site content with the content found in this package" to install the theme's starting point. This will remove your site's existing content and replace it with the theme's starting point. Select "No" if you do not want to remove your site's existing content.