To Upload Videos you can either use SwitchTube and link or embed the Video here, by clicking on the „Add Media“ button in the editor mode of the chapter.
Then you can add your video from your private library by selecting „Upload Files“ –> „Select Files“
It will then be saved in the „media library“. There you can click on the video you want to add to your chapter and click „insert into chapter“.
You can remove the video from the chapter by clicking on the x-icon or edit it (for example adding subtitles) by clicking on the pen-icon. (These icons appear when clicking on the video when you are in the editor mode of polybooks.)
For completely removing the video select „Delete Permanently“ in the media library.