Videos can be inserted from YouTube or Vimeo - all you need is the link to the video.
To insert a video
Click inside the article where you'd like the video to appear
To the left of the flashing cursor will be a grey '+' button - click this
From the menu that appears, click Video
In the popup window that appears, paste the link to the video
Click the Insert button and the video will be inserted
To update which YouTube or Vimeo video an embedded video points to
Find the video on the page and click it to select it
Click the Edit button that's over the top of the video
Paste the new link into the popup window that appears
Click the Update button and the video will be updated
To remove an embedded video
Find the video you wish to remove and click it to select it
Press the backspace key on your keyboard