We'll need to know what kind of video card or onboard video you are using. Go to Device Manager and look at Display Adapters and tell us what it says there.
Most people are going to use Nvidia or AMD/ATI video cards, but onboard video can be Intel. There are a few other choices, especially in older computers, but these are the main ones.
To update the drivers, you should go to the online site for whichever video hardware you are using and download them. Installation is simple, you just start up the installation program that you just downloaded and its pretty much automatic from there. You'll need to reboot the system afterward.