IIRC there will be set of 8 switches by the CPU. The manual, if you have it, should tell you what to set it for a given speed.
I don't know if all were similar but switch 6 would be the clock multiplier and 7 & 8 would set the Cpu clock speed. I think, if all this fits your MB, 100Mhz would be off/off/on and the next available speed would be 120Mhz which would be on/on/off.
I doubt that it would be stable at 120Mhz and would advise not to bother as it really isn't going to be worth it. I definitely wouldn't try it based just on what I recall and I will not take responsibility if something goes wrong.
Your best bet is to check the manual or get a second opinion.