How do you change the clockspeed of a voodoo/voodoo2 exactly??
Is it done by the Pll in the ramdac or does it work another way.

For Voodoo2 cards you could use Voodoo2 Overclock tool: this one is on line at Voodoo2/Tweakers section.

I wanted to know how this was done EXACTLY in hardware.

why do you want to do this "in Hardware" when you can do this clicking with the mouse[?]