Expire user password and force reset at next login in Linux

To cause a user password to expire you can use passwd (-e or --expire to force it to expire):
sudo passwd -e user
Another method is to use chage (using -d or --lastday command which sets when (in days) the password is last changed. The parameter may also be expressed in the format YYYY-MM-DD):
sudo chage -d 0 user
sudo chage -d 1970-01-01 user