Time limit to edit posts is 30 days.
If changes are required after a month, then a mod will be happy to change it for you.
so you can't return and update as appropiate, it requires an edit by a mod (which is what i did say in a previous post).
You can within the 30 day period yourself. Just click on modify
and you get the option to edit your post and the title.
What it may not always do is change topic title for posts made by others throughout the whole thread.
You can see that various posters have managed to edit the topic title themselves. ie psychopomp, daniel, skyeci, tiffy etc