passwd command

Users in Linux have a password assigned. You can change the password using the passwd command.

There are two situations here.

The first is when you want to change your password. In this case you type:

passwd

and an interactive prompt will ask you for the old password, then it will ask you for the new one:

When you're root (or have superuser privileges) you can set the username for which you want to change the password:

passwd <username> <new password>

In this case you don't need to enter the old one.