Don’t try to change it in Roundcube. Roundcube is just a front end for your email. Change it in Virtualmin.
- Choose the domain for which you would like to change the user’s password. …
- Click Edit Email and FTP User’s .
- Click the name of the user who’s password you would like to change.
- Next to Password , click the Set to radio button.
- In the textbox next to Set to , enter the new password.
- Click Save .
Try that and see how it goes.