You should make a copy of the template before starting, if you want to be able to roll back any changes. In this way, you will be able to go back to the original and copy the code there if you mess up. Also, you might overwrite your modifications without a copy, if you use the default folder and at some point upgrade your Joomla installation. You will now have two copies of the template on your Joomla website. You will have to edit the template XML file to name the copied template to something else. Template overrides override the output of Joomla’s core modules and components, without requiring you to hack the core. This will allow you to override the output of the content component that shows the content of the website. Your overrides are left untouched when the Joomla core is updated. The files might be overridden by a future Joomla update, if you modify the same files in the core. Template overrides can also be used for installed modules and components.