When I try to use sudo apt-get update, I get the following error:

```
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.

```

what can I do to update Ubuntu correctly? 

I'm on Ubuntu 17.04