Open The Sims 4
Load your save file
Press Ctrl + Shift + C on PC or open the cheat console on console
Type testingcheats true and press Enter
Type cas.fulleditmode and press Enter
Close the cheat console
Hold Shift and click the Sim you want to edit on PC
On console, use the interaction menu on the Sim
Select Modify in CAS
Edit the Sim in Create-a-Sim
Save your changes and return to the game