You can use Head & Shoulders shampoo to strip the colour from your hair. When you wash your hair with it, leave it on for 20-30 minutes, and then once you rinse it out, soak your hair in conditioner because this shampoo can dry your hair out. This method is quite inexpensive and will quickly take the dyed colour out.
If you cannot find Head & Shoulders, a clarifying shampoo will often do the trick as well.
My hair is naturally blonde and I dyed it to a dark brown (that had dark auburny undertones as a result of my natural blonde) a few months ago. Since then, I've box dyed it that same color just to keep it fresh looking and keep my roots at bay. Now I've decided to go back to blonde, and I've been asking everywhere for help. This is what I've gotten: Highlights are the best option if you are wanting to go back to blonde. You can do all over highlights repeatedly (every few weeks perhaps, but I'm not a professional, so don't quote me) until you reach your desired color. Bleaching it is VERY harmful to your hair and although you may reach the color you wanted, your hair will be damaged and gross for a long time. I agree that it's best to consult a professional about your specific type of hair and what is best for you.
Quite simply, you can't...do not try this yourself...you will be spending money on something or other that could have gone to a professional who will do it right the first time! This is nothing to mess with on your own~!