Hello Imran,
Sergiy is absolutely right. A dirty trick I can suggest to change password history count for all users is to change the value for all users directly in database:
UPDATE USR SET PASSWORD_COUNT ='10' WHERE SITE='SITE_NAME'
.... this should update Historical Password Count field to 10 for all users on the site.
Obviously, this is not a supported way, but... 
regards,
Konstantin