Need help with class change?

This may seem like a dumb question, but I'm having trouble with switching back to my main class. I have a level 200 sorcerer, and I switched to a level 140 phantom mage just to try it out. As I said, I want to switch back to that level 200 sorcerer, but I don't know how to. I haven't played in so long. If anyone can tell me where to go, that'd be awesome!

Thanks!