Upgrading from Debian Squeeze (6.0) to Debian Wheezy (7.0)

Debian Squeeze was released 06.02.2011.
Please note that Wheezy is still in testing -stage.  Debian Wheezy was released 04.05.2013

Just some quick steps how to do the upgrade (on your own risk).

Update Squeeze

aptitude update
aptidude upgrade

1. Copy /etc/apt/sources.list to /etc/apt/sources.list.d/debian-wheezy.list and replace squeeze with wheezy. Or copy them inside sources.list. Use whatever mirror which is closest to you.

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

2. Update repository

   aptitude update

3. Upgrade critical parts first – it will complain about libept1 – just let it be removed.

   aptitude install dpkg apt aptitude

4. (Dist-)upgrade rest

   aptitude upgrade
   aptitude dist-upgrade

5.  IF YOU ARE STILL RUNNING UNDER XEN3.x:
   Replace grub2 with grub1 (or just keep your old menu.lst at /boot/grub/)

   aptitude purge grub-pc
   aptitude install grub-legacy

6. Always check that /boot/grub/menu.lst or /boot/grub/grub.cfg exists and defaults to right kernel
7. Reboot and hope for the best

<complete instructions>
https://www.debian.org/releases/stable/i386/release-notes/ch-upgrading.html
</complete instructions>