I have Ubuntu 16.04 LTS and I've installed some things in the initial setup, but it was a lot, and I don't remember in which steps I messed up in order to apt get update
give this error:
Get:48 http://archive.ubuntu.com/ubuntu xenial-backports/universe amd64 DEP-11 Metadata [4.712 B]
Reading package lists... Done
W: GPG error: http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04 Release: The following signatures were invalid: KEYEXPIRED 1496576244
E: The repository 'http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04 Release' is not signed.
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.
How can I fix these errors?