Well, first of all, you will have to see if your butterfly is capable of any flight. No butterfly can fly at 40 degrees Fahrenheit, so you will have to wait until things get warmer and sunnier, meaning next spring, if she lives that long (which she might). If she is able to fly, then you can release her when it is warm (in the spring). If her wing is so damaged she cannot fly, then you basically have a pet on your hands.
Give her the sugar water, or better yet honey water (because honey is made, of course, from actual nectar). Use 1 part honey to 4 parts water. Make sure it is clean, so that no particles will clog her proboscis. Pour it into a rough, shallow dish (rough so that her claws can find some rest). If she does not drink, you will have to manually unroll her tongue with a toothpick (put it in the "hole" in the middle of the circle, and just draw it out until the tip is in the fluid). You will have to hold her, with wings closed over her back, while you do this.
You... all... stare... but you'll never see, there's something inside me