You should download and install the latest version. (RC4) All versions prior to 1.6 are discontinued and for the most part unsupported. Older versions also have security issues and tons of bugs.
You can download the latest release here: http://www.osticket.com/downloads.php(http://www.osticket.com/downloads.php).
Unfortunately you can not upgrade from 1.3 to the latest version due to the DB/code changes. However, you can install the current version and write a small script to migrate the tickets.