1. Add the following to the bottom of the SHOWTHREAD template, right before .
2. Now anyone who has permission to do so (admins, mods, and the thread starter, if you have it set up like that) can double-click the thread title within the first post and edit it. (Just like you can do on forumdisplay)
3. If you want the clickable/editable element to be something other than the first post title-- for example in the breadcrumbs or a custom title at the top of the thread-- then just add id="thread_title_XXX" to that element, replacing XXX with the thread id. Make sure the element you add the ID to is wrapped by some other element, such as a div, because the parent element is actually what becomes clickable.