Main page slider

How am I able to edit which posts are seen on the main page slider. My dev tried diving into your code and couldnt figure why some images are being shown and some are not since the posts are duplicates…. We went in and uploaded 2 different images to the posts and only 2 showed up but also I want to be able to have the slider show recent posts. Please can you help. My dev partner is really skilled and if he can’t figure it out I’d like to try and get your help.