How can I dye/bleach my already dyed hair?

So a few months ago like really long ago I went to a hair salon and dyed my hair to a more lighter brown, my original hair color black/super dark brown. So it was slightly bleached I think, but didn't get the color I wanted which was a dirty blonde so I ended up with a dark caramel color. But I haven't had my roots touched up or anything for MONTHSS and I would really love to dye my own hair to a dirty blonde, but I just don't know how to do it exactly and what products I should get first, and since my roots are black would that mean I would have to leave the bleach there for a longer period of time or?... Advice would be really appreciated!