I expect they'll be stored in the ini file which will likely be /etc/config/twonkyvision-mediaserver6.ini
You can use SSH (and PuTTY) to navigate to that directory, copy it to one of the shares and open it with Notepad++ (free download) which better 'understands' Unix format files (the end of line code is slightly different from Windows files, so best not to use Microsoft's built-in Windows Notepad). You can either edit it (to remove the password) and then copy it back (to overwrite the existing one), or you can simply delete the original, then restart Twonky (it should build a new one automatically; it uses the default.ini file to build the replacement). If you have any unforeseen problems after deleting it, you can simply replace it with the one you've copied to the share.
If you remove the password then put the file back, there will be no other changes. If you decide to delete it instead, you will have to go into Twonky config and apply any custom settings (the media paths, server name, rescan time, etc).
If you are unsure about PuTTY and SSH, have a look at my post here
. That post shows how to open PuTTY and copy a file to the NAS (it's actually for changing the music trees) but the principles are exactly the same. In your case, you could do the below.
Log in using PuTTY (as shown in that post) then type the following commands:
cp twonkyvision-mediaserver6.ini /share/Web
Leave the PuTTY session open, then use Windows to look at the Web share and Notepad++ to open and edit the twonkyvision-mediaserver6.ini file, then go back to the PuTTY session and do the following to copy it back
cp /share/Web/twonkyvision-mediaserver6.ini .
(the dot after the space means 'to here').
Restart Twonky and it should now be password free.