help from hairdressers

She won't be able to "dye" it back to blonde; dye can't lighten dye. Your hairdresser should use a colour remover (like ColorFix, Color Charm Color Corrector or Igora Modulat) to remove the reddish brown. She may suggest just bleaching, but ColorFix is much more gentle. The ColorFix (or similar product) should remove all the reddish brown from your hair. If there's any colour remaining, a quick bleach should do the trick. She should then tone your hair to your desired shade of blonde. (If you have any roots showing, that would be great -- she should be able to use the natural colour of your roots as a guide for the new colour and get as close a match as possible.) If she tells you this isn't possible, don't let her touch your hair. Go to someone who specializes in colour correction. Don't let yourself be talked into a more expensive, complicated process. Fixing your colour shouldn't be very difficult. Good luck.