If the Sim is old enough, if you go to where their backpack is, click on the cell phone. An option that says something about houses will be their, click on it. And then, if you look around the screen, you will see something about moving out.
Beware! Make sure you're sure you want to move out before you do, because once you move the Sim in to another house, the other Sims will no longer be playable.