#update /etc/apt/sources.list to refer to squeeze and not lenny:
deb http://ftp.uk.debian.org/debian/ squeeze main non-free
deb-src http://ftp.uk.debian.org/debian/ squeeze main non-free
deb http://security.debian.org/ squeeze/updates main contrib
deb-src http://security.debian.org/ squeeze/updates main contrib

#update apt repository
apt-get update

#update apt dpkg aptitude first
apt-get install apt dpkg aptitude

#perform a dist-upgrade
apt-get dist-upgrade