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I just checked by editing a thread I started 9 mths. ago. After you click on edit thread you must go to the title line & click so you get the typing bar. You can then backspace or delete the title & then type in what you want. Assuming it's a thread you started!
As SB said... it has to be a thread you started... you can't edit somebody else thread... If it's your thread do as SB said.. open the thread.. click on "edit thread" which is right under the title of the thread... then click in the title box and backspace to clear the title ... now put in the new title...