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. :-)