Going from brunette to blonde is a big change that's a little more complicated than dyeing a lighter color to a darker color. Bleaching the hair lighter than desired, then adding warmth with the appropriate darker color and any desired highlights is time-consuming, but generally produces the best results. Good aftercare is also essential, as lightening the hair can cause damage.
The first thing to do is to choose the dye. Look at guides online or consult a stylist. Hair colors are divided into two categories: warm and cool with a range of undertones. It is important to choose colors that work well with the natural tones of skin and eyes, as these cannot be changed.
This will be the lightest shade within the range. Dye the hair with this color to lighten it a couple of shades. Wait until the next day to complete the next step, to prevent excess damage being done to the hair.
Choose a shade slightly darker than the initial step to neutralize the ultra-blonde look. This will make it look more natural. If the color is now too dark, add blonde highlights to lighten the overall tone.
Dyeing your hair can cause damage. Use a moisturizing shampoo and a good quality conditioner, and be careful when blow drying, straightening or curling, as heat can do additional damage to hair. Schedule frequent salon visits to get professional advice on how to care for dyed hair.