Editing user accounts and their permissions is a role reserved for System Administrators. To change a user's password in Movable Type, you follow the same steps as editing a user's profile:
Switch to the System Overview area of Movable Type by clicking on the blog selection menu and choosing "System Overview."
Click Manage > Users from the System Overview's main menu. This will open a listing of all users that exist in your system.
Locate the user whose password you wish to change and click on their username, which will open the User Profile screen. Here you can change their password, as well as edit basic profile information about the user, optionally create a new personal blog for the user, and optionally assign the user system administrator privileges. See Editing a User's Profile for more information.
1 User Contributed Notes
It's probably worth noting here that passwords are only significant through the first eight characters.