Change Windows 7 64 Bit Computer To 32 Bit?

I want to play a game called ace of spades, But when i try and start it, it says that it doesnt work with the version of something that i have, and it says that i need to figure out wether or not my computer is a 64bit or a 32 bit. It's a 64 bit. but my brother has the same game and it works on his computer, and we have the same computer. HE says, the game works on 32 bit computers. SO i need to find out how to change my 64 bit computer to a 32 bit compter.