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