Look at your thread ...
Your name is on left - in line with that is ...
The topic title in the centre - in line with that is ...
The modify button on the far right beside the 'quote' button .....
BUT you are only able to modify your headings and content for 24 hours after posting. After that, contact a moderator who will do it for you. I presume the modify button disappears after 24 hours which is why you can't see one.
This is one of the newish changes to rootschat and has been around for about a year or so. I think the reasoning is so that people don't go back over their posts and alter them so that any replies no longer make sense. I can see the logic in that, but it does give the moderators more work to do, and prevents small alterations such as adjusting spelling errors which did come in useful if noticed over 24 hours after posting.