Upgrading from Ubuntu 16.04 in late 2021

I got access to a computer using Ubuntu 16.04. Since it is out of support, I am in need to upgrade it to, say, 20.x.

To do this, I am prompted to execute sudo apt-get update before. But this gives the following error messages:

Reading package lists... Done
W: The repository 'http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Ubuntu_16.04  Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://cloud.r-project.org/bin/linux/ubuntu xenial-cran35/ InRelease: The following signatures were invalid: KEYEXPIRED 1602869253  KEYEXPIRED 1602869253  KEYEXPIRED 1602869253
W: Failed to fetch https://cloud.r-project.org/bin/linux/ubuntu/xenial-cran35/InRelease  The following signatures were invalid: KEYEXPIRED 1602869253  KEYEXPIRED 1602869253  KEYEXPIRED 1602869253
E: Failed to fetch http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Ubuntu_16.04/Packages  404  Not Found [IP: 195.135.221.134 80]
W: Some index files failed to download. They have been ignored, or old ones used instead.

I assume that some have-a-look-see-URL became invalid. (especially probably http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Ubuntu_16.04/Packages) What is / are the correct URL(s) and how to teach them to apt-get?

Until now, I have not saved important data / programs on the HDD, therefore backups are not relevant.

Additionally, using an external medium is not allowed.

PS: This post was created on this site thanks to @john-jones.

EDIT: Replacing security.(...) and de.archive.(...) by old-releases.(...) did not work, since the stem does not refer to a site (here) containing xenial-xerus (16.04). Going up via "Parent directory" and going to xenial-xerus (here -> here -> here) did not yield a page containing a folder named "Packages" or something similar. I am confused. :-)

You have to launch Software & Updates (or software-properties-gtk command) and then disable http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Ubuntu_16.04 from it. Or use text editor. This repository is no longer exists.

Please also make sure that your /etc/apt/sources.list contain the following lines:

deb http://archive.ubuntu.com/ubuntu/ xenial main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ xenial-updates main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu xenial-security main restricted universe multiverse

And remove ESM list by sudo rm /etc/apt/sources.list.d/ubuntu-esm-infra.list to prevent unexpected behaviour.

To restore R GPG key you have to run the following command:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys E298A3A825C0D65DFD57CBB651716619E084DAB9

Then you have to:

1. update package lists by

   sudo apt-get update
   

2. install all new dependencies for 16.04 LTS existing system using

   sudo apt-get dist-upgrade
   

3. remove R packages to prevent their possible conflict on upgrade to 18.04 LTS using this answer (steps 1 to 3).

4. reboot

5. set your release upgrade way as LTS to LTS using commands below:

   sudo sed -i "s/^Prompt=never/Prompt=lts/" /etc/update-manager/release-upgrades
   sudo sed -i "s/^Prompt=normal/Prompt=lts/" /etc/update-manager/release-upgrades
   

6. run distribution upgrade to 18.04 LTS using sudo do-release-upgrade or update-manager -c .

7. reinstall R packages on 18.04 LTS using this answer (steps 4 to 5)

8. reboot

9. run distribution upgrade to 20.04 LTS using sudo do-release-upgrade or update-manager -c .

10. reboot

11. enjoy