I have a YouTube account and I saw a very nice playlist but unfortunately it is not well ordered. I would like to save that playlist and edit it.
Could someone tell me how this can be done (saving the playlist) in the new YouTube interface?
On the playlist page, click the "+ Save" button to save the playlist to your library as-is. A playlist saved like this will get updated as the original creator updates it. You can also clone the playlist by clicking the three dots > "Add all to..." button and adding all videos to a playlist you fully control.
The old design is currently available by appending
&disable_polymer=1 to the URL, or by using Internet Explorer. It may disappear in he future though.
On the watch page, you can save the playlist by clicking the 3-line icon with the plus next to it.
On the playlist page, you can save the playlist by clicking the very same icon in a different position.
Note that these options merely save the playlist to your library, they don't clone it for you to edit it. To clone playlists, you'll have to find a bookmarklet or an API program that basically iterates through all items of a playlist given playlist and creates a new playlist from your account. Creating a program like this is
left as exercise to the reader out-of-scope for the Webapps Stackexchange and more something for StackOverflow. Though this exists if you trust an unknown third party application to take over parts of your account.