@DuncaGallagher Both. If you have testingcheats enabled, you can shift-click on a sim and Modify in CAS. Some other mods like MCCC will allow you to modify the household in CAS.
If you only want to change hair or makeup, you don't need cheats enabled or MCCC for that. You can do that on any mirror. And you can use any CC you have installed there as well.