If you don't have root access to the server and/or your host hasn't installed any additional templates, then you will not be able to change templates.
The default squirrelmail installation is located at /var/www/html/squirrelmail
You can change the current theme of squirrelmail from one of the options page. A list of available (installed) themes will be presented in a drop-down box. There are several included in the standard installation, however most of them are simply color changes.