vi /etc/apt/sources.list #replace content

deb http://ftp.de.debian.org/debian/ squeeze main contrib non-free
deb-src http://ftp.de.debian.org/debian/ squeeze main contrib non-free

# squeeze-updates, previously known as 'volatile'
deb http://ftp.de.debian.org/debian/ squeeze-updates main contrib non-free
deb-src http://ftp.de.debian.org/debian/ squeeze-updates main contrib non-free

apt-get update
apt-get upgrade
apt-get dist-upgrade

vi /etc/apt/sources.list #replace content

deb http://ftp.de.debian.org/debian wheezy main contrib non-free
deb-src http://ftp.de.debian.org/debian wheezy main contrib non-free
deb http://ftp.de.debian.org/debian wheezy-updates main contrib non-free
deb http://ftp.de.debian.org/debian-security wheezy/updates main contrib non-free

apt-get update
apt-get upgrade
apt-get dist-upgrade

reboot

vi /etc/apt/sources.list #replace content

deb http://ftp.de.debian.org/debian/ jessie main contrib non-free
deb-src http://ftp.de.debian.org/debian/ jessie main contrib non-free

deb http://security.debian.org/ jessie/updates main contrib non-free
deb-src http://security.debian.org/ jessie/updates main contrib non-free

apt-get update
apt-get upgrade
apt-get dist-upgrade

reboot

vi /etc/apt/sources.list #replace content

deb http://httpredir.debian.org/debian stretch main contrib non-free
deb http://httpredir.debian.org/debian stretch-updates main contrib non-free
deb http://security.debian.org stretch/updates main contrib non-free

apt-get update
apt-get upgrade
apt-get dist-upgrade

reboot