How to upgrade Debian6 Squeeze to Debian 7 Wheezy? Make a backup before upgrade Debian  vi /etc/apt/sources.list replace with below: deb http://mirrors.kernel.org/debian/ wheezy main deb-src http://mirrors.kernel.org/debian/ wheezy main   deb http://security.debian.org/ wheezy/updates main deb-src http://security.debian.org/ wheezy/updates main   # wheezy-updates, previously known as 'volatile' deb http://mirrors.kernel.org/debian/ wheezy-updates main deb-src http://mirrors.kernel.org/debian/ wheezy-updates main apt-get update apt-get dist-upgrade check debian version via command "lsb_release -a"Distributor ID: DebianDescription:    Debian GNU/Linux 7.1 (wheezy)Release:        7.1Codename:       wheezy