how do i return to the original OS

in desperate need to fix my rotary encoder/jog wheel

im going to have to buy a 2000xl rotary and install it. however the fault is that it skips by 2 instead of one.

either way, i have the jj os 3.12 and i want to revert back to the original OS so that the rotary will work...

im confused to what the internet is telling me

so i thought i'd come here for a garunteed explanation

thanks a million guys

www7a.biglobe.ne.jp/~mpc1000/update308.htm