You can edit NPC Sims in your game and give them a makeover by using testing cheats. Open a cheat window by pressing Ctrl + Shift + C on your keyboard and then typing testingcheats on in the cheats window then pressing Enter. Then type another cheat called cas.fulleditmode in the cheat window and press Enter again. Press Ctrl + Shift + C again to close the cheat window. When you see a Sim you'd like to edit do a Shift + Click on that Sim and then click on "Modifiy in CAS". From there you can make the changes to that Sim and click Play when you are done.