Did you install youtube-dl with pip or apt? I'd really recommend against
using apt because the version in the repositories is really outdated and
that's not good considering how often Google updates the API. If you
installed it through pip, you can run:
sudo pip install --upgrade youtube_dl
If you installed it through apt, you can run:
sudo apt remove youtube-dl
and follow the instructions at
https://ytdl-org.github.io/youtube-dl/download.html, that is
sudo wget https://yt-dl.org/downloads/latest/youtube-dl -O
/usr/local/bin/youtube-dl
sudo chmod a+rx /usr/local/bin/youtube-dl
Then you'll be able to update simply with sudo youtube-dl -U
