Pour a small amount of fingernail polish remover on dried nail polish. Blot the stained area dry with a towel. Do not rub, as the polish can spread to other areas when you are using the remover.
Pour some powdered oxidized bleach on the stain, then use a wet toothbrush to scrub out the stain. Oxidized bleach is safe on colors, so you don't have to worry about it damaging the fabric of your couch.
Wet a towel and add a small amount of laundry detergent to it. Create a small amount of lather in the towel, then scrub the stain with it. Allow the fabric to air dry.