I had a similar issue on Ubuntu even after installing Nodejs many times with the latest version, it was showing always the same old Nodejs version; I discovered it was installing the similar old Debian package each time, even after executing the apt-get update command
Finally, I got it to work by purging the old nodeJs then adding different repository source, and installing nodeJs normally with the new distribution as follows:
sudo apt-get purge --auto-remove nodejs
curl -fsSL https://deb.nodesource.com/setup_14.x | sudo -E bash -
sudo apt-get install -y nodejs
Please find the list of all NodeJs distribution below
https://github.com/nodesource/distributions/blob/master/README.md
You may find other ways of doing the update, but this one worked for me.