Since you are using the Newscast theme and not Corona the process is different. In Newscast, the slideshow area is controlled by the Mainpage options in the theme options. Its the set of options at the very bottom of the wordpress navigation on the left hand side of the wordpress admin.
There you'll see the options for Mainpage Slideshow including what kind of Slider to use, how many slides and most important Featured Slider Categories. This is where you'll see which post category gets put into the slider. So when you go to Posts, you'll be able to add new posts to replace those in the slider.
If you need to download new documentation, you can just download the theme again from Themeforest under your accounts Downloads.