Instructions
Go to the the main neighborhood screen. Press Control, Shift and "c" simultaneously. A text box should appear in the upper left-hand corner.
Type "BoolProp Testingcheatsenabled true" into the box (without quotation marks) and press Enter.
Select the lot where the Sim whose name you would like to change lives. Once the game is in play, click on the Sim while pressing shift. Make sure this is the active Sim by noting the box around his picture on the left.
Look for the option "spawn" from the listed items, and click. Afterward, click on "Tombstone of L&D." A tombstone should then appear.
Click on the tombstone and press "rename sim," and type in the new name. To bring the game back to normal, as in take off the cheat mode, re-enter the cheat code in Step 2 but with "false" in place of "true."