Welcome to the Ranch. If you were just looking to do a bulk update you could do it with a JPQL update statement
Of course when you do this you are going around the entity manager factory so you might want to clear your persistence context by calling clear() on the entity manager afterwards.
For batching there is no good way to do this with JPA that I know of. You are going to have to drop down to provider specific stuff. For Hibernate see the below link.
https://docs.jboss.org/hibernate/core/4.0/devguide/en-US/html/ch04.html
Of course your next question is going to be how do I get the Hibernate Session if I am using JPA?
Try this (assuming you are using JPA2):
Also I am moving this to the ORM forums.