Getting Back to Natural Hair Color After Dyeing?

I am naturally a platinum to almost ashy blonde and I wanted something different and went to a salon, dyed my hair by keeping the blonde on the bottom and medium brown on top for an ombre look and it's been about 4 months. I didn't get it cut then, but 4 months prior. My blonde roots are coming in and its beginning to look weird having blonde on the bottom a light brown in the middle and blonde roots so I would like to go back to blonde fully, but was not sure how to do that. I don't have a problem with the salon doing it!