我使用 Ubuntu 16.04 访问了一台计算机。由于它不再受支持，我需要将其升级到 20.x。
为此，系统会提示我sudo apt-get update之前执行。但这会给出以下错误消息：
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.
我假设一些“看看”URL 变得无效。 （尤其可能http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Ubuntu_16.04/Packages)什么是正确的 URL 以及如何教它们apt-get？
到目前为止，我还没有在硬盘上保存重要的数据/程序，因此备份不相关。
编辑：此外，不允许使用外部介质。