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 maindeb-src http://mirrors.kernel.org/debian/ wheezy main deb http://security.debian.org/ wheezy/updates maindeb-src http://security.debian.org/ wheezy/updates main # wheezy-updates, previously known as 'volatile'deb http://mirrors.kernel.org/debian/ wheezy-updates maindeb-src http://mirrors.kernel.org/debian/ wheezy-updates mainapt-get updateapt-get dist-upgradecheck debian version via command "lsb_release -a"Distributor ID: DebianDescription:    Debian GNU/Linux 7.1 (wheezy)Release:        7.1Codename:       wheezy