Change Sims appearance

When you click on them you'll see this:

Just remember it shouldn't work on people your Sims work with or Sims of other players. 

It won’t show up ;-;

Is your game fully updated?

Pretty sure it is

Could you share a screenshot of your CAS and one of the Sims that doesn't show this button but should in your opinion?

Here

Could it be you're below level 12?

Maybe

If you are, you unlock the makeovers at level 12, if you want to see what you unlock when you can click on the level bar at the top of your screen.