How do i temporarily take control of a sim? (sims 3)?

It should be quite simple but first you must find out where she lives in the town because you'd have to switch households for a moment to change her appearance and stuff. WHile playing as your current household go to edit town. Then on the left hand side of the screen there'll be a on with two houses on it. Click on that and it'll ask you if you want to switch active households. Click yes. Now Click on select household and click on the household with the girl in. You can now play as the girl and change her appearance. When you want to go back to playing as the household with the boy go back to edit town and switch active households. Then select the boy's household and play as him again. Hope this helps :)