Sigue las instrucciones que vienen en el .zip:
Upgrading from a previous version of vBulletin 3
Close your board via the admin control panel.
Upload all files from the 'upload' folder in the zip, with the exception of install/install.php and includes/config.php.new
Open your browser and point the URL to
http://www.domain.com/forum/install/upgrade.php (where
www.domain.com/forums/ is the URL of your vBulletin). You should now be automatically forwarded to the appropriate upgrade script and step.
Follow the instructions on the screen. Make sure you click "next step" or "proceed" until you are redirected to your admin control panel. Here, you can reopen your board.
For a complete description of how to upgrade from a previous version of vBulletin 3 to the latest version, see the upgrade section of the vBulletin 3 Manual.
http://www.vbulletin.com/docs/html/upgrade
En resumen:
1.- Cierra el foro, desde el panel de control.
2.- Copia todo el directorio upload que viene en el zip, salvo install/install.php y el includes/config.php.new
3.- Ejecuta el upgrade que hay dentro del install:
http://www.domain.com/forum/install/upgrade.php
4.- Suerte
Salu2