Hi. I am playing fallout 3 on my laptop and the game doesn't run that good. Does anyone know how I change the directx?

You just download and run the directx installer from Microsoft.

If you're on Steam it might try to update it if you try to use an older version of directx, but there usually isn't a reason to do that.

He is clearly asking if he can tell his PC to use dx 9 or 8, alot of well designed games can do that now, because not all PCs have top notch graphics cards. ie Laptops.

The game already uses DX9, it doesnt use DX10/11. You are only able to change the quality of the games main settings.
OP:What hardware do you have?

If you have any graphics card that came out after December 19, 2002, then it will support DX9, or August 13, 2003 if we are talking specifically 9.0b, so this shouldnt be an issue for anyone. What is more then likely the issue is the OP doesn't have the hardware, IE, an Intel chip.