How do you remove hair dye from your hair so it goes back to the bleached color?

So i’ve dyed my hair alot, but each time i have to wait till it fades and that takes a couple of months. And i see a lot of scene girls especially that change there hair so quickly (like every week or 2 weeks) and they remove all of the color first. But i don’t get how they do that. So my question is how do you remove the hair dye quickly? (i use semi)