With this Chrome addon for instance: https://chrome.google.com/webstore/detail/switch-for-youtube/jnhjlfocfbjeofbbbiideliepnhkhmml
Or
Go to Youtube.com
At the end of the url paste: ?disable_polymer=true
Optionally make a bookmark.
Yes you are rightthanks @HoNeYBiRD, that is an important feature they took away on the new version, i wonder why they did it