I was trying to get rid of grays and used a light ash brown & came to dark how do I lighten it?

Nothing at all. The level should eventually fade to the color it described, or at least close to it. Just use leave it in to process for less time next time, or don't use that shade NEXT TIME. But I wouldn't try anything chemical to get it to go lighter. At least it is in the brown FAMILY which I am sure is what you wanted.