the post above is right im just adding an alternative.
In Odin town/castle w.e its called there is an NPC that allows you to change your class at lvl 30. She also allows you to change classes at lvl 40-45 I cant remember which. Se is at the north east part of Odin castle if i recall correctly.
Doing the what is said on the post above will bring you to her, but just in case u ever get lost that is where she is.
here is some screenshots of what has been posted.
Odin Castle Map: