You can navigate to the post that you turned into a global banner by using the edit button.
Then, click the wrench above the timeline or at the bottom of the topic to open the topic menu, select “Un-pin topic,” and then “Remove banner topic.”
In case you dismissed the banner and don’t remember which topic you pinned as a banner, you can pin any other topic as a banner. You can only make one topic a banner, so this overwrites the current banner, and then you can unpin the new banner.