If you need to change password to a lot of users, you can use this command line tools:
With dsquery, you can filter the users by name and/or OU.
With dsmod, you can change the password.
On the example I change password to all users that start with exit80 and are inside a specific OU (200_Users). Remember to change mypassword with the desired password.
dsquery user -name exit80* "OU=200_Users,OU=100_Users,DC=Domain,DC=GRP" | dsmod user -pwd mypassword