If you have forgotten your password, you cannot access your BSCW workspaces any more and, of course, you cannot change your password as described above.
For exactly this case of emergency, BSCW provides a specific procedure to assign a new password without having to provide the old one:
Open the registration page of your BSCW server (on FITs public BSCW server this is http://public.bscw.de/pub) and click [reregister].
Enter one of the email addresses which BSCW has already associated with your user name (e.g., your primary email address) and submit the form.
BSCW will send a message to that address, which contains the URL of a form that allows you to enter a new password.