From the vault window, use Change master password. You'll need to re-enter and verify your current master password first; Vodou then re-encrypts the entire vault under the new one, with a fresh cryptographic salt.

If you've genuinely forgotten your master password rather than just wanting to change it, see "Forgot your master password?" in Setting up the password vault — because the password is the actual encryption key, that path deletes the vault rather than recovering it.