Please make sure to back-up your store database and files before template installation over the existing store just in case.
1) To install OpenCart template you need to upload it’s files to your hosting server. Open OpenCart template package, find "themeXXX(update).zip" archive and upload it to the root of your OpenCart installation.
The template files won’t harm your store in any way.
2) Unzip the theme archive on your server.
3) Type – yourwebsite.com/install/ – in the address bar of your browser and follow the instructions on the screen.
If you are missing any elements after installation, just clear your browser cache.
4) Please make sure to delete the "install" directory from your hosting server for security reason.
Feel free to check the detailed video tutorial below:OpenCart 1.5.x. How to install a template over the existing store (using "update" package),