First of all you have to find the theme that you wish to install. Once you have downloaded the theme on your local PC you will need to extract the files.
Once that has been done you can upload the files via FTP to your OpenCart folder and you will have to replace the current admin, catalog, image and system with the ones from your theme.
After that you have to navigate to the Administrative area of your website and go to Extension -> Extensions and once there select Modules from the drop-down menu.
You will see the name of your theme and click on the install button and after that you will have to go to the drop-down menu once again and click on Themes.
You will have to click Edit on the theme from the drop-down menu, click save and you will have completed the installation of the desired theme.