If you want to change something in a core Joomla file, say the layout, heading tags, etc., their is a proper way to approach this. If you alter the core files you run the risk of breaking your site if done improperly and if you upgrade your version of Joomla, this file is overriden.

You need to copy the original php file from the core and place the copy into the proper place in your template directory. The correct directory structure for your override file is:

TEMPLATE_NAME/html/EXTENSION_NAME/VIEW_NAME/FILE_NAME.php

If you wanted to override the core file that controls the way an article is laid out, copy the original into the new directory inside your html directory of your template files and edit it from there. Say for example you are using the protostar template. Use the following structure:

Original File location: JOOMLA_ROOT/components/com_content/views/article/tmpl/default.php

Override File Location: JOOMLA_ROOT/protostar/html/com_content/article/default.php

Joomla 3.2 does this work for you

Pull the Extensions menu down, click on Template Manager   Extensions -> Template Manager

You will come to a chart of the templates that are installed on your site. The columns are Style, Default, Location, Template and ID.

There should be two templates where the Default star is filled in, one is your Administrator Template the other is the Site Template.

Click on the name of the default site template in the Template column.

This brings you to the Template Manager: Customise Template screen.

Click on the Create Overrides tab.

Screenshot of the Create Overrides template

We are assuming that you have done your research and know what file you wish to override and how to edit the file to accomplish your goals.

Say you wish to change something about the default way an article is laid out. Click on com_content and all of the views for that component will be displayed.

Click on Article, and after a moment, you should get a green box on top that says that the Override was created and the path to that file. This file will be place in the /templates/TEMPLATE_NAME/html/ directory.

Click on the Editor tab.

Click on the html folder in the left hand column.

This will open up to the existing sub-folders.

For our example of wishing to edit the Article layout, click on com_content -> article -> default.php.

Keep in mind that this is a copy of the core file. If you totally mess it up, you can simply delete the file from this directory, and Joomla will go back to using the original core file.