You can't change it but the game actually runs better on 64 bit. I am on windows 8, 64 bit and my game does not freeze. The game has system requirements. If your game is freezing, does your computer meet the requirements to play the game? Do you use CC or Mods? What world are you playing?
http://simswiki.info/wiki.php?title=Game_Help:TS3_System_Requirements
