Editing your video after uploading it to YouTube is nothing new, but that would result in a new video with a new address, and YouTube guessed not many users would want to go through the hassle of linking their videos all over again.
Well good news is YouTube has released a new video editing tool. The magical "Edit video" button will allow you to add effects, swap soundtracks and trim your clips. After all, we wouldn't have time to do those if we posted the videos right off our mobile phones do we?
The previous video editing tool, YouTube Editor, will still be around, but if you want simplicity, the one-click "Edit video" option might work for you.
Source: YouTube blog