Do you mean editing the video itself or things such as tags and descriptions? If you are referring to editing a live video, the answer is that you can't do it. However as blackzp said, you can edit the other stuffs such as tags and descriptions. From my experience, making changes in the description (for a highly ranking video) has always resulted in a drop depending on the extent of the changes. At times the video may bounce back to its original position or simply stagnate at a lower position. Honestly, I wouldn't risk it for a video ranking in Page 1 of Google.