kann nicht mehr updaten. Dies ist die Meldung:

sudo apt-get update
[sudo] Passwort für user:
Ign:1 http://de.archive.ubuntu.com/ubuntu zesty InRelease
Ign:2 http://de.archive.ubuntu.com/ubuntu zesty-updates InRelease
Ign:3 http://de.archive.ubuntu.com/ubuntu zesty-backports InRelease
Fehl:4 http://de.archive.ubuntu.com/ubuntu zesty Release
404 Not Found [IP: 141.30.62.23 80]
OK:5 https://repo.skype.com/deb stable InRelease
Fehl:6 http://de.archive.ubuntu.com/ubuntu zesty-updates Release
404 Not Found [IP: 141.30.62.23 80]
Fehl:7 http://de.archive.ubuntu.com/ubuntu zesty-backports Release
404 Not Found [IP: 141.30.62.23 80]
Ign:8 http://security.ubuntu.com/ubuntu zesty-security InRelease
Fehl:9 http://security.ubuntu.com/ubuntu zesty-security Release
404 Not Found [IP: 2001:67c:1360:8001::17 80]
Paketlisten werden gelesen... Fertig
E: The repository 'http://de.archive.ubuntu.com/ubuntu zesty Release' does no longer have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://de.archive.ubuntu.com/ubuntu zesty-updates Release' does no longer have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://de.archive.ubuntu.com/ubuntu zesty-backports Release' does no longer have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://security.ubuntu.com/ubuntu zesty-security Release' does no longer have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

sudo apt-get upgrade
Paketlisten werden gelesen... Fertig
Abhängigkeitsbaum wird aufgebaut.
Statusinformationen werden eingelesen.... Fertig
Paketaktualisierung (Upgrade) wird berechnet... Fertig
0 aktualisiert, 0 neu installiert, 0 zu entfernen und 0 nicht aktualisiert.

Was kann ich denn machen?