If you would like to change the default Joomla! favicon.ico then visit the Favicon Generator web site.

The favicon generator site allows you to convert any jpg, png or gif file format into a favicon.ico file for your Joomla! site.

Just visit the favicon generator web site, upload a gif, jpg or png file 16px by 16px and the web site will generate the favicon.ico file for you which you can then upload to your Joomla! site.