Most of the time you can just uninstall the driver from add / remove programs in control panel. Reboot the machine and load the new driver. On a few problem updates, I have used a utility called driver cleaner. It deletes all references to the old driver.
You will need to know the brand of chip used in the card. Most cards today are Nvidia or ATI. Both mgfs have reference drivers on their sites. You can also generally download a driver from the card mgfs site; although quite a few of them just link to nvidia or atis site.