Browse to System → Access Control. Click on users and use the edit icon to view this users details.

Fill in the two password forms and confirm by clicking update.

The password is now changed, if the user was previously logged-in using the old password, (s)he will be able to continue to browse the HSMX management web-interface. Once their session ends (log out/time-out), they'll have to re-authenticate using the new password.