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.
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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.: