To upgrade vBulletin version one need to perform below mentioned steps:
1. Download the latest version of vBulletin in zip file by login to your vBulletin Member’s Area.
2. Take Full Backup including database of your vBulletin site through web hosting control panel.
3. Disable the board by login to vBulletin admin panel.
4. Unzip the latest downloaded vBulletin file.
5. Upload the contents to the root directory or the specified location of the forum
6. Run /../install/upgrade.php file.
7. Enable the Board.