m (JaPurity how to center hornav moved to Customising the JA Purity template/customisations/Centering the hornav: Moved page to a subpage of Customising the JA Purity template/customisations for consistency)
The original JA Purity template was provided from JoomlArt.com as a zip but the latest files have been installed along with Joomla. The relevant files, index.php and ja-sosdmenu.css are in the following directories:
templates/ja_purity index.php css/ ja-sosdmenu.css
If you've been following the tutorial you can download a tutorial version of the template that installs to the following directory:
templates/my_japurity index.php css/ ja-sosdmenu.css
You can edit the file directly in your choice of plain text editors such as VI or Notepad or you can use the HTML Editor provided in the Joomla Administration area.