The way I do it is to simply install the driver over the old one. I've never had any problems with this.
The right method is to unistall your old drivers first via 'Add/Remove programs'. Look for nVidia display driver. Your PC will reboot and upon startup, windows will default to a basic video driver (Your screen will look wierd becuase of low resolution and colour). Then you simply install the new driver. It will reboot again, and upon restart the desktop will be at a low resolution but you can change these settings via desktop properties.