Navchandi yagna is usually a Nav Durga puja. This puja is to help make needs come correct and likewise for health and fitness, wealth, strength. Also the prosperity, results and all kinds of other complications in the existence. Saptashati indicates chanting of 700 Shloka, describing the importance of Goddess Durga, https://www.youtube.com/watch?v=QIX0JZlLsgE