While the editor is pretty basic, the main advantage is that you can replace the original video, but this only works if your video has less than 1,000 views and YouTube hasn't received a copyright complaint from a third-party. There's also a "save as" option that lets you save the edited version as a new video.
How to find the new editor? Make sure you're logged in, go to "My videos & playlists" and click "Edit video" next to the video you want to edit. You can also open a video you've uploaded and click "Edit video".
To make things more confusing, YouTube has another video editor that lets you mix your videos and Creative Commons videos from other YouTube users, add audio soundtracks from YouTube's library and apply transitions.