How to use Creative Commons licensed videos on YouTube? According to YouTube help center, Creative Commons licensed videos can be reused and edit. 
But it is not allowed to download videos from YouTube, so how can i edit it?

YouTube Studio beta only allows to edit your uploaded videos.
As Dr Mayhem mentioned, if you can see it and it has an appropriate license, you are allowed to reuse it.

I would recommend also as they suggested to ask the author first if they can send you a copy somehow. Alternately, I think youtube-dl should have you covered for downloading directly. It's a command-line tool so you will have to use the shell of your OS (e.g. Command Prompt in Windows) to use it, but it's fairly easy to use and has a plethora of options for getting the exact quality you require. Generally you can probably get by with just calling it without any parameters e.g.:


  youtube-dl www.youtube.com/watch?v=xxxxxx