Upgrade to whatever the latest version available to you is. There is additional documentation on upgrading in the vBulletin Online Documentation:
http://www.vbulletin.com/docs/html/upgrade
But the basic process is to upload the contents of the "upload" directory from the latest version you download off the Member's Area; then run the Upgrade script by going to:
http://yoursite.com/forum/upgrade.php.
The process is fairly straightforward after that.