How can change secondary profession?

I started Warrior and secondary profession Monk. But now I see the skills aren't very good, and instead I looked up on WIKI and found that Assassin would work very well with my Warrior. How can I change my secondary profession from Monk to Assassin? And can I do it for my Warrior that is on Prophecies?
I am lvl 10 now.

You can change your secondary in Prophecies after you are "ascended" and talk to the NPCs in some outposts in the desert. But that's only for Core professions.

In order to change you secondary to Assassin, you would need to own Factions and be able to get to Senji's Corner in Kaineng City.

So basically, don't worry about your secondary just yet. Just concentrate on your Warrior primary and only use the Monk secondary for a better rez. Leave the healing to the Monks (humans or henchmen).

EDIT - oops, forgot about the profession changer in GToB, (see below)

So what level is that "ascended" at? Because I'm already half way through to 20...

It's not a level, but a place. If you have Prophecies, you need to do all the missions in the desert and defeat your doppelganger before being able to change secondary.

Ascended

Secondary Profession

There are some good PvE wammmo builds, but TBH I only use my monk secondary for ressing anyway. The process of "ascention" is a plot device, not a level acheivement and since your in Proph you will know when you are "ascended" the plot line makes a big deal about it. On the other hand in Night Fall I didn't realise towards the end that I had already ascended.

Don't worry to much about getting assassin skill just yet. Most of those W/A builds are designed for hardmode and elite areas. These places are designed for characters that have beat the entire game anyway. You will do just fine as a wammo for Proph and can switch to w/a when you hit factions. Then go to NF and get some go w/p or w/d skills for Eotn then. Then worry about builds for the elite areas and hard mode.

You must have Factions to be able to unlock Assassin as secondary profession, but you don't need to play through half of the Factions campaign to actually unlock it. After you have ascended in Prophecies (this is a requirement) you can visit the profession changer who is present in the Great Temple of Balthazar. Once you unlock a secondary profession for the first time, you can change to all of your unlocked secondary professions in every outpost freely.

You are able to visit the Temple as soon as you reach one of the harbor cities while playing through the main story line. For Prophecies that is Lion's Arch. From there you are able to visit the other campaigns as well as the Temple.