change an existing password for the current user

passwd

change the existing password for the root user

sudo passwd

change it for a specific user

sudo passwd alice

delete the password for the specified user

sudo passwd -d alice

This will allow the user to change the password without specifing the “current” password, however they will not be able to login without a password

expire a password for the specified user

sudo passwd -e alice

lock an account

sudo passwd -l alice

unlock an account

sudo passwd -u alice