In this tutorial, you will see how to manage OpenCart 3.x modifications.
Usually, modifications are required when you install the template over existing store, re-install them or simply add new ones. Modifications are extending default engine functionality without changing OpenCart core files.
Installing the template over existing store, you need all the modifications that are located in the themeXXX_extensions.zip archive. Extract the archive first, then go to the Extensions > Installer tab in your admin panel. Upload modifications one by one and, after doing so, go to the Extension > Modifications and on the top right click Refresh modifications button.
In case you need to re-install the modification, you need to delete it prior to installing it under Extension > Modifications menu as you will get an error saying that this modification already exists. After that, install it under Extensions > Installer tab.
Visit your site and check for the changes.