Doh, I´m sorry. Do not figure OUT was that.
Basically, GO to mail section (yourdomain.com:2030/index.php?module=mail_list)
Click ON THE NAME of the mailbox you wanna edit.
Edit or/and Save your new password.
As i have see now, root admin can change password and see it.
Anyway, if you have a passwd you will advert about the change.
No matter if you wanna hide passw from admin, it can log into ANY user account then set email again, and SEE it. For the most paranoids, you must know that emails can be readed in a plain text format with NO AUTHORIZATION *, because that layer of security is email server based, not filesystem based. Only PGP (2.6 or above) signed emails will be safe from a sysadmin.
* This happends in MOST hosting panels. Emailing is not an encrypted thing.