Here are the official repositories for Linux Debian Jessie (8.0).
This should be entered in the file: /etc/apt/sources.list
# Debian Jessie
deb http://security.debian.org/ jessie/updates main
deb-src http://security.debian.org/ jessie/updates main
deb http://ftp.at.debian.org/debian/ jessie main contrib non-free
deb-src http://ftp.at.debian.org/debian/ jessie main contrib non-free
To make sure the GPG key is also saved locally to avoid Key error messages issue the following commands.
gpg --keyserver pgpkeys.mit.edu --recv-key CBF8D6FD518E17E1
gpg -a --export CBF8D6FD518E17E1 | apt-key add -
apt-get update
Now to upgrade to Jessie:
apt-get dist-upgrade