Jan 202013
 

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>