Seventh day Prayers for Navratri is dedicated to Goddess Kalaratri. Navratri Day 7 Mantra in Hindi is given below. There are three mantras. First is Mool Mantra for seventh day of Navratri. Second is Dhyan Shloka dedicated to Goddess Kalaratri. 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.