The only way to do it, is called "Forcing a Moult" and it is as brutal as it sounds. You keep the chickens mainly in the dark, with hold feed and water and they will shed their feathers, and regrow them. But I would just say give her time, lots of sun, access to fresh greens/bugs and all that other good stuff and she'll moult soon on her own.
Oh, and I'm not sure about chickens, but when my parrots feathers start to look ragged, I give her a bath. Cleans her, and then she preens her feathers nice and tidy afterwards. Can you give a chicken a bath? If you can and its warm enough out, chances are she'd preen herself in the sun afterwards and maybe look a little less ragged. Just a thought.