1. Backup your database.
2. Download a new vBulletin package, unpack it and upload the files, overwriting your old ones. It would be best if you empty out your vBulletin directory first.
3. Open
http://www.yoursite.com/install/upgrade.php and follow the instructions.
4. Once done, go to admincp and revert whatever templates it asks you to revert. It would be best to revert all of them.
5. Delete install folder.