I've done this. Its actually quite simple! Buy a hair color remover. That will strip it and leave it orangey. Then my suggestion is to dye it an ash blonde. That will get rid of the dramatic orange. Next - BLEACH. Then if that is what you want, excellant. If you are looking for a particular blonde, buy that and put it on your hair. I suggest doing all this within two weeks, but only if you feel your hair can take the beating. Your roots will grow out if you wait to long. You need to get to the color you want as soon as possible.
Oh and, CONDITION LIKE A MOTHER. Just leave it in, don't wash it out and do not brush with a comb, use your fingers. Don't blow dry if you can. Pamper it alot because its just been beaten to death, lmfao.