Right now, there is no way to edit the description of a video after you upload them. This means that adding changes to the hashtags, mentions, and title of the video can't be done yet.
But as a workaround, the best way to add new hashtags and mentions easily is to add hashtags and mentions in comments, as seen below.
If this isn't going to work for you, and you feel the need to update the description, the steps to follow are:
Delete the video from Plays.tv. It's under the menu on the right edge of the video frame.
Go back into your Game Sessions or Saved Clips and upload the video again, with a much more desirable description, hashtags, and mentions.