apt update error with ubuntu 18.10 cosmic version

The repositories are still available on http://old-releases.ubuntu.com/ubuntu. You can change them in /etc/apt/sources.list, replacing all occurences of http://archive.ubuntu.com with http://old-releases.ubuntu.com.

After that, apt-get upgrade will work again.

More info on dealing with no longer supported versions at https://help.ubuntu.com/community/EOLUpgrades

Finally I forced the downgrade to change the source.list ... I followed this article : https://linuxconfig.org/how-to-downgrade-ubuntu-linux-system-to-its-previous-version

1. It's because your Ubuntu version is out of date. That means Ubuntu does not provide security support because the system is too old. You need to disable the bad repos in your sources list and then run apt again.

2. apt-update does not upgrade your system. It only updates the repositories in your system. If you need to upgrade to another Ubuntu version, run do-release-upgrade. Otherwise, run apt-upgrade -y to upgrade system packages. Obviously, run sudo if you are not the root user.