Passwd: Only root can specify a user name
When Changing a Password using passwd command only the root user can specify a username. Which means only the root user may change a password of a another user.
If a regular Linux user wants to change their own password, just type passwd command without specifying the user name.
But there is a one way, which non root users also can able to change the password of a another user. That is, using sudo command. But for that user needs to have sudo privileges.