在Ubuntu 16.04中更新我的软件包列表时，如何解决该错误？

尝试更新Ubuntu 16.04时出现此错误。我已经尝试过很少的解决方案，但到目前为止没有任何效果。

错误：

W: http://archive.canonical.com/ubuntu/dists/precise/Release.gpg: Signature by key 630239CC130E1A7FD81A27B140976EAF437D05B5 uses weak digest algorithm (SHA1)
W: http://us.archive.ubuntu.com/ubuntu/dists/precise-security/InRelease: Signature by key 630239CC130E1A7FD81A27B140976EAF437D05B5 uses weak digest algorithm (SHA1)
W: GPG error: http://extras.ubuntu.com/ubuntu precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 16126D3A3E5C1192
W: The repository 'http://extras.ubuntu.com/ubuntu precise Release' is not signed.
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: http://us.archive.ubuntu.com/ubuntu/dists/precise-updates/InRelease: Signature by key 630239CC130E1A7FD81A27B140976EAF437D05B5 uses weak digest algorithm (SHA1)
W: http://us.archive.ubuntu.com/ubuntu/dists/precise-backports/InRelease: Signature by key 630239CC130E1A7FD81A27B140976EAF437D05B5 uses weak digest algorithm (SHA1)
W: http://us.archive.ubuntu.com/ubuntu/dists/precise/Release.gpg: Signature by key 630239CC130E1A7FD81A27B140976EAF437D05B5 uses weak digest algorithm (SHA1)
W: The repository 'http://ppa.launchpad.net/upubuntu-com/network/ubuntu xenial 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.
E: Failed to fetch http://ppa.launchpad.net/upubuntu-com/network/ubuntu/dists/xenial/main/binary-amd64/Packages  404  Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.

从软件和更新部分中取消选中了一些不需要的软件包/存储库后，我设法摆脱了一些错误，但仍然仍然存在少量错误：

 The repository 'http://ppa.launchpad.net/upubuntu-com/network/ubuntu xenial 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: http://archive.canonical.com/ubuntu/dists/precise/Release.gpg: Signature by key 630239CC130E1A7FD81A27B140976EAF437D05B5 uses weak digest algorithm (SHA1)
 E: Failed to fetch http://ppa.launchpad.net/upubuntu-com/network/ubuntu/dists/xenial/main/binary-amd64/Packages  404  Not Found
 E: Some index files failed to download. They have been ignored, or old ones used instead.

应用以下提供的解决方案（由Raphael提供）后，大多数错误得以解决。除了这些错误：

 W: The repository 'http://ppa.launchpad.net/upubuntu-com/network/ubuntu xenial 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.
 E: Failed to fetch http://ppa.launchpad.net/upubuntu-com/network/ubuntu/dists/xenial/main/binary-amd64/Packages  404  Not Found
 E: Some index files failed to download. They have been ignored, or old ones used instead.