How do I get the The Stallman hat. It mentioned edit a community wiki post, but I edited some post which is bumped on the main page like the screenshot below and have not yet received that hat. Do I understand it wrongly? If so, where can I find the community wiki post?
That is not a community wiki post. That is just a post which has been bumped by the Community user. See this question for more information on Community Wiki posts. A community wiki post is a post that the author of an answer thinks should belong to the community and not to them. Questions can also be converted to Community Wiki but only by a mod or dev. The user who posted the Wiki post doesn't get any rep for upvotes or lose rep for downvotes. Anyone can change it given they have 100 rep. An example of a community wiki post is the formatting sandbox.