This is an item I've been asking for years now, it'd be nice if there was an option on the thread menu for the creator or a mod/admin to click that would mark the thread as resolved, other forums have this feature but the thing is that it's not a vBulletin feature, it's an addon to the forum of which Neal is hesitant to add because it's likely to break with each software upgrade and it's not supported by vBulletin. Maybe some day in the feature we'll see it here too, but for now if you're not able to edit the first post in your thread(s) you can either ask a mod or post in the thread stating that it's resolved.