For many years, the YouTube video editor, also known as the YouTube editor was an ideal tool you could use to edit videos that had already been uploaded to YouTube. The good thing about using the YouTube editor is that you get to keep the video URL and maintain the views and SEO juice after the edit.
YouTube has announced that the video editor, together with the Photo slideshows feature, will be going away on September 20th 2017. There is a banner about this on the YouTube editor page.
Fortunately, most of the features of the YouTube editor will not go away. They will be moved to ‘Enhancements’ which can be accessed from the video manager. So from September 20th editing a YouTube video will require you to go to your video manager select the video you want to edit and from the enhancement options you can choose whatever you want.
YouTube have not yet said anything about YouTube slideshows apart from the fact that it’s going away.