Usually to change some elements of iOS system, we would expect to be achieved through the jailbreak, such as changing the shape of iOS folder.
Recently, some users have found a way can change the iOS folders into a circular, you only need to change the iPhone wallpaper to achieve that.
YouTube user videosdebarraquito show two methods, which involves a process to set the home screen wallpaper. According to the video demonstration, you need to download a small enough image (video used in the 3×3), then enter the Photos app to set this image as the main screen wallpaper.
If you do not have 3×3 wallpaper, you can also go to this site to get some ready-made materials. This page shows the “round Folder” tips should support iOS 9.3.1, 9.3 running on any iOS devices. If you do not like these ready-made wallpapers, you can crop any image according to your preference, the premise is wallpaper should be small enough.
At present we do not know what its “principle”, but according this seems does not change the shape of all your folders. If you try it on the iPad, some folders will turn round, but some still are square, which depending on the orientation of iPad.