You just want to lighten the hair to a lighter shade of blonde?
Just bleach it, but not completely. Don't let it set the whole time required. You can check the hair as it is bleaching to see when the color you desire is there. If the idea of bleaching scares you, and depending on how long ago you dyed your hair, you can do a standard dye job with light blonde hair dye. If you just dyed it recently, it is best to wait at least 6 weeks to redye your hair or it will fry it.