add and change featured images to slider (free version)

Hello, I can't find out how I can change or add new pictures (sites) to the slider. I had it with 4 featured pages. Then I deleted one and added another page to my website which I want to be featured as well but even tough the slider No. is on 4 it only shows the 3 pages I had in there from the beginning. I can't even remember how I got them in the slider at the first place. I hope you can help me with my problem. Thank you in advance.