...You need to enter TestingCheats, shift-click and press "Edit sim in CAS". Of course, this only works if you have Generations, or the Generations patch...
Just to clarify, the entry sequence (the way I use it) is:
1) In Live mode (I believe you can be in Pause), press
Shift-Control-C simultaneously to open the blue cheat bar (top of the screen).
2) Type "
testingcheatsenabled true" (without the quotation marks), then press
Enter.
3) Press
Esc to exit the cheat bar.
4) Make the sim you want to change the Active Sim (in the list of sim portraits on the left of the screen).
5)
Shift-click the sim him/herself in the game-screen to bring up special options, including Editing in CAS and Modifying Traits.
You do NOT need to have Generations or ANY of the EPs/SPs or any CC. I have none of that, just the base, vanilla Sims 3. And the sequence I described works for me.
@Simstar3
The CAS cheat is a 'newly' sanctioned cheat by EA (but some of us have been doing it a while). It is a very glitchy cheat. Rule of thumb...
1. NEVER change age, weight, or anything in the Basics tab.
That's it. Don't do it. Its only good for changing the way a sim looks.
If you do, your sims will be horribly disfigured. If you want to try and save the sim, maybe take it back into CAS and return it to its rightful age 
I'm sorry to hear your sims have had disfigurement problems. That has to be very frustrating. I have to agree with Chronic, however. While I haven't changed age or basic body structure, I
have fiddled with muscle definition and skin tone and haven't had a problem yet. Maybe it's just the age tweak that causes problems?
Anyway, as always, one should always save before making any major changes. Learned that one the hard way. I didn't save before giving a sim Steel Bladder (a Wish I wish I'd skipped).
