A favicon is a small, square image that appears on the browser's tab (if the browser supports tabbed browsing) and on the left side of the site URL in the address bar. The default icon is the Joomla! logo, but you may change it to suit your site.
By default, the Joomla! favico.ico file can be found in the /images directory. Some templates, however, will contain code that redirect the browser to /templates/template_name/icon. To determine where your new favicon should be, examine /templates/index.php and look for code that resembles: <link rel="shortcut icon" href="http://yoursite.com/templates/your_template/icon/favicon.ico" >
2. Convert to ico format using free online sites such as: http://converticon.com/, http://www.favicongenerator.com/, http://www.htmlkit.com/services/favicon/, http://tools.dynamicdrive.com/favicon/, http://www.favicon.cc/
3. Copy it to the path outlined above.