You don't. Pinning a post in this (or generally any forum anywhere) is done by the Administrator(s) of the forum.
Send them a polite message asking for the post in question to be un-pinned. They will typically unpin the post or send you a reply explaining to you why they didn't
Pinned posts are pinned because they are of enduring "first-look" importance to all readers of the forum in question. One request to un-pin (or to pin in the first place) a forum thread will not be acted on. A "whole bunch" of requests likely will be.