check on the motherboard if there aren't any jumpers (packs of them). Or use a software overclocking (softFSB, CPUFSB, ...). Try to find a diagram of your motherboard that tells what every jumper or connector is, and if you do, try to find the settings for anything.
open the case, on the motherboard should be printed instructions on how to set multiplier, look for 4.5x and find the jumpers you need to set to get it like that. if that works set it to 5x, 5.5x or 2x (it acts like 6x) but i boubt yo wil get over 100MHz increase