Here you go:
(please do that in textmode and not in your GUI, so KDE or whatever can be restarted without any issues)

• vi /etc/apt/sources.list
  (change lenny to squeeze)
  Should look like this:
  

  deb http://ftp.nl.debian.org/debian squeeze main contrib non-free
  deb http://security.debian.org squeeze/updates main contrib non-free
  deb http://ftp.nl.debian.org/debian squeeze-updates main contrib non-free
  

• aptitude update
• aptitude install apt dpkg aptitude
  (accept the first solution, it is usually fine)
• aptitude full-upgrade
  (read through the suggestion, but the first one is usually the best)
• aptitude upgrade

enjoy!