Short answer: You can't! (easily)
Long answer:
because the password is also the "password" for the encryption used by Voyager this is not easy to do.
The only way so far is:
- either backup your mail using the built in backup function or export your mail to the mbox format (I'd recommend that one)
- Backup the whole voyager directory as well (to be on the safe side), so you can go back to this if everything fails.
- delete the whole voyager from your stick
- reinstall voyager and use a new password
- reimport your backed up mail

And yes, this sucks!