Ubuntu Upgrade from 16 to 18

 1) https://www.liquidweb.com/kb/how-to-upgrade-ubuntu-16-04-to-ubuntu-18-04/

However, sudo do-release-upgrade not working.

2) go to a second link

https://askubuntu.com/questions/1205332/do-release-upgrade-failing-on-ubuntu-16-04-lts-says-no-new-release-found

1. Before, I had modified my default /etc/apt/sources.list file to the one described here: https://gist.github.com/rohitrawat/60a04e6ebe4a9ec1203eac3a11d4afc1 (that step was probably useless, since my sources.list file looked fine). After that, do-release-upgrade still produced a "No new release found" message.

2. I changed xenial to bionic in /etc/apt/sources.list (there was nothing to change in the files under /etc/apt/sources.list.d/) by running this:

   $ sudo sed -i 's/xenial/bionic/g' /etc/apt/sources.list

3. I upgraded everything with:

   $ sudo apt update && sudo apt -y dist-upgrade

4. And reboot