How to change URL structure

Hi friends, One of my friend is using vbulletin 2.8.5 version. Can any one tell me how to change its URL structure..??

Present its URL is like this : Site.com/forum/xu12.PHP
I want to change that as : site.com/forum/post-title.php or site.com/forum/post-title.html

Please guide me in changing the URL structure.

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