This Powershell script let you change all AD Users Password in a certain OU. With the parameter -mustchpwd you force users to change their password at next logon.

1.) Copy the Powershell Code below
2.) On your Domain Controller, paste it into ISE Powershell and run it

If you need to know your LDAP Path you can check this with ADSI Edit (Search it from Windows Search Button)

For more Information about DSMOD visit Microsoft Technet