You can edit your thread title by editing the title of the first post or by double-clicking next to the thread title in the forum overview. This is only possible a few minutes after creation though.
After that period you have to ask a moderator. You can do that by using the report button (
) which is located on the left side below your post or by sending a private message to a moderator of the respective section (or to a global moderator in case there is no local moderator).
You can find the list of local moderators on the right side below the list of threads in the forum overview or here: