This is a simple procedure how to upgrade Debian 6 squeeze to Debian 7 wheezy. This is aimed to systems that doesn’t have any custom manual tweaks or installed software that it’s not from the package manager.
First thing you need to do is make backup of your /etc directory. The upgrade process will NOT modify anything in your /home directory. Also for this upgrade apt-get is the preferred method not aptitude.
For those who want more detailed procedure you can read the official upgrade procedure from here.
Update Debian squeeze
Get the latest updates for Debian squeeze:
apt-get update apt-get upgrade apt-get dist-upgrade
Upgrade to Debian wheezy
Change the source list file to the new Debian version and make the upgrade
sed -i 's/squeeze/wheezt/' /etc/apt/sources.list apt-get update apt-get dist-upgrade -d apt-get upgrade apt-get dist-upgrade
Now you can reboot into your Debian wheezy system
Check your Debian version
You should get this output
No LSB modules are available. Distributor ID: Debian Description: Debian GNU/Linux 7.0 (wheezy) Release: 7.0 Codename: wheezy
That is it, now you have your system upgraded from Debian squeeze to Debian wheezy
Debian Website: http://www.debian.org/