Overclocking your graphics card is done through the OS. You should have a graphics utility that came with the card. It may have any number of names depending upon card manufacturer, 'Afterburner', 'SmartDoctor', 'OCGuru', etc.
If your motherboard does not have on-board graphics, do not get too aggressive in your advancements. As soon as graphics issues start appearing, it's time to back off
Hope it helps some