From Online Manual
There are three ways for a moderator to rename a topic. The first two methods have a limitation regarding the subject line of replies in the topic. The third method does not have this limitation:
1. Modify the first message in the topic display and edit the subject line. However, this only affects the root message and any new replies posted to the topic. Any replies posted prior to editing the first message will retain their original subject line.
2. This method requires quick moderation to be turned on. On the message index, double click in the space alongside the subject line that needs to be changed. The topic title is displayed in a box which can be edited. This has the same limitations as modifying the first message.
3. If the forum has a spare or private board, move the topic to this board and choose the option to Change the topic's subject. Additionally, choose the option to Change every message's subject, and then move the topic back to the original message board.