In bios go to "CPU PnP Setup page"
It depends on which motherboard and processor you are using. Processor can be overclocked by BUS multiplier and BUS Speed.
e.g 3.0 GHZ processor (200 * 15 =3.000 GHz)
200 is BUS speed and 15 is BUS multiplier. Intel processors are BUS multiplier locked. Only option for INTEL processor is to change BUS speed, if your motherboard support.
Remember, over clocking generates more heat, without adequate cooling you may fry your processor. There is no warranty for over clocked dead processor.