How to upgrade my linux Debian installation?

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http://unix.stackexchange.com – I run the following command to upgrade my system - aptitude update aptitude dist-upgrade However it gives me updates for the softwares and libraries that I have installed. This is not what I want. I want to upgrade the operating system itself, to apply any security patches or bug fixes. What is the way to do that? I am running Debian 6.0 Squeeze. (HowTos)