Debian: How to upgrade from Squeeze to Wheezy?

First step is to be sure that your Squeeze is up to date:

apt-get update && apt-get upgrade

Next step is to have proper entries in sources.list file. Edit file by typing:

gedit /etc/apt/sources.list

and replace the whole content with:

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

You can change country code to a different one if needed. I’ve used servers from Poland. Now update sources:

apt-get update

Than upgrade packages:

apt-get upgrade

As the final step we upgrade the whole distribution:

apt-get dist-upgrade