You'd need to have testingcheats on enabled and cas.fulleditmode cheat activated. It takes going into close up mode (click on the jaw) and then manipulating the jaw down a little. Sometimes it's almost impossible to remove completely so it may be better to choose a different jaw template altogether. We can't actually manipulate directly the saggy bit that makes it look like the sim has a double chin but sometimes by manipulating the jaw and making the neck smaller it does make a slight difference.
To be honest, when I see a sim like that I just go through the jaw templates with the sim in CAS with the sim in profile, find a jaw line that looks chiseled and then take it from there.