Wash it well with shampoo and conditioner about every other day (any more and you're stripping your hair of much needed oils and such) and it should fade within two weeks at most.
A lot of the time when you dye your hair the color will come out a few shades darker than it said on the box, ways to prevent that? You can experiment but I've never really cared so I don't actually know- a first for me as far as hair coloring goes.
I do know the only way to chemically lighten your hair is to bleach it, or find some shampoo (if it exists) that strips color treated hair. But again you risk lightening your natural hair color more than you intended and once it's lighter it won't go back, you have to wait for it to grow out again.
You could always just dye it a different color... like light brown or something, it won't come out exactly as you want it too but it will look lighter for sure. Or if you don't want to use hair dye use toner! orrr put in some light brown highlights ;) it will lighten your hair while giving it dimension
Yeah that's what happens when you dye your hair dark brown, it will fade used head and shoulders shampoo and wash it thoroughly it will probably take about 2 months to lighten up. In my experience guys seem to like darker hair on pale girls.