Posts can only be edited for a certain amount of time after they were submitted. If there is something specific about a given post that needs to be changed (such as to remove passwords or other sensitive information you did not intend to post), use the Report Post button ("!" inside a triangle) to alert the forum's moderator(s).
"Please give us a simple answer, so that we don't have to think, because if we think, we might find answers that don't fit the way we want the world to be."
~ Terry Pratchett in Nation