Click on the setting icon located in the lower right corner of the Youtube player, then select the speed you want.
But the problem is may be you don't see the Speed menu at your Youtube player.
To be able to display the speed menu you need to make enable YouTube HTML5 player.
Go to http://www.youtube.com/html5
Then click on the Request the HTML5 player
Re open the Youtube video you want to adjust the speed. The speed menu will appear now, if not, may be the video is not supported.
