From Online Manual
There are several ways for a moderator to rename a topic:
1. Modify the root message 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 root message will retain their original subject line.
2. In the message index, double click in the space alongside the subject line that needs to be changed. The the topic title is displayed in a box which can be edited . This has the same limitations as modifying the root 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.