Well the bios usually picks the correct multiplier setting on setup. This is usually automatic.Look for something like frequency\voltage control and see if there is any reference to the multiplier settings.
Usually they are manually controlled by a series of jumper switches on the motherboard. The default is usually auto. You can manually configure this to the settings you want. Look in the motherboard book for multiplier switches. On most boards there are a bank of on/off switches.