I searched the FAQ and got the following:
If you have registered, you will be able to edit and delete your posts. Note that the administrator can disable this ability as he desires. Your ability to edit your posts may also be time-limited, depending on how the administrator has set up the forum.
To edit or delete your posts, click the button by the post you want to edit. If your post was the first in the thread, then deleting the post may result in the removal of the entire thread.
After you have made your modifications, a note may appear, which notifies other users that you have edited your post. Administrators and moderators may also edit your messages but this note may not appear when they do so.