Whenever the news content is being refreshed the featured images are updated automatically, still if you want to add image/video into slider manually please follow below steps.
Step 1: Click on Content Option in left menu [Folder Icon] and click on Posts
Step 2: Click on Add New Post
Step 3: Select Website in which you want to add slider image/video
Step 4: Select Post type as Video for Video Slider or Image for image slider slide.
Step 5: Select Category and subcategory
Step 6: [IMPORTANT] select Featured, this will mark that post as featured and it will list inside slider.
Step 7: Save
All the changes made by you will get reflected on website. if you face any issue please let us know.