Ninth day Prayers for Navratri is dedicated to Goddess Siddhidatri. Navratri Day 8 Mantra in Hindi is given below. There are three mantras. First is Mool Mantra for eighth day of Navaratri. Second is Dhyan Shloka dedicated to Goddess Siddhidatri. Third is a Stotra. You can chant also prayers like Lalita Sahasranama on the day.
There is no need to chant all mantras. Most devotees chant just the mool mantra.