From Online Manual
There are several ways of renaming a topic, but they do not all work as might be expected:
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 background of the subject will change, and it can now be edited. This has the same limitations as modifying the root message, explained above.
3. If the forum has a "spare" or private message board, move the topic to the spare board and choose the option to Change the topic's subject. Additionally, choose the option to Change every message's subject. Then move the topic back to the original message board.