1. The first thing you need to do is to log in to your site admin page.
2. Then go to Apperance>> Themes.
3. Now you will see all the themes you have currently installed, to add another theme click on Add new button.
4. Select and install a theme from the official Wordpress theme directory.
5. Simply click the Activate button to activate the theme.
For further details go through this blog
https://www.siteground.com/tutorials/wordpress/install-themes/