is your computer a custom built, or did you get it at a store (Dell, HP, Compaq, eMachines, ect)? If you custom made it, then you go into the BIOS when your comp starts up (usually by pressing F2, F11, DEL) and go to CPU configuration, i cant say for sure what to do there, cause all motherboards are different, but there is a spot to change the speed.
Although, if yours is a brand name, then you cant overclock it, they do something to the BIOS so you cant, that way there not responsible if you mess it up. Only way to get around this is to buy a new mobo, and you would probably need a new case also.