you can read in here:
http://noscrubs.net/discussions/a-newbie-s-guide-to-ns-iris-online



or for faster

6.7. Job Promotion

At Level 30 you get a new quest from Shilly 'New Job' that will lead you to your class instructors (you can see what kind of skills each class has at those instructors). Class instructors will then give you a quest to change class.

At Level 75 you get a new quest from Sedy/Galadriel NPCs to job change.