I just tested this on one of my posts on a different thread. Here is wahat you do:

1. Locate your post that you want to edit.
2. Click on the "Edit" button"
3. You will notice a Quick Reply section with the body of your post in the text window. Locate and click on the "Go Advanced" button.
4. This will take you to another screen that you can edit your title.

I hope this helps you do what you need to do.

THanks.