Joomla facivon can be changed by using ftp to upload a new favicon.ico image to your templates/mytemplate/ folder. If this doesn't work, go to your site and do a right click -> view page source. Using control F in most browsers will open a 'find' tool, enter favicon. This should show you where your templates favicon is located, ftp the new favicon to this location. You can change this location by editing (not advised) or doing a Joomla Overrite for the template file. More information on Changing the Joomla Favicon.