Change my secondary proffesion

So I created an warrior, started in Nightfall (I liked the model better).
But I want Assassin as my secondary proffesion, but at level I didnt had the choice of getting that one.

How can I change it to assassin?

Sorry for the newbie questions, but I just got the new expansions 2 weeks ago.

I don't know where you have to get to in Nightfall to switch to Factions/Prophecies, but once you do you just have to complete the quest to get to Canthan. Once there you have to go to Senji's Corner and there is a Profession Changer. It costs 500gp and you can change your profession any time you want.

I believe once you have built your base you can change secondary as a nightfall character. Essentially after the first primary quest is done once you get to the mainland then get the profession changer.