First, login as root or an admin group user. To prompt for and set a local user's password, use the /etc/scripts/user-mod command. For example, to change operator1's password:
/etc/scripts/user-mod -P operator1
This command also works to reset the root user's password.
Full details on managing local users from the CLI, including setting passwords non-interactively (e.g. for automated scripting), can be found in the article Managing local users from the command line.