You own Password
You can change you own password with
passwd. You will need to enter you current password, before you can set a new one.
Password of another User
Root can reset other users passwords. You will not be asked for the current password and can set a new one.
[tux@server]$ passwd <username>
Force User to reset Password on Login
You can use the
chage command to force a user to reset their password. Normally
chage set the expiration date of the users password, but if you set it to 0 the use has to change it immediately.
[tux@server]$ chage -d 0 <username>