As with the post of How to upgrade Debian Etch to Lenny, I actually know this may be know by a lot of people already, but here is how I did it, and if you ever forget can come here and remember it.

First edit your sources.list

sudo vim /etc/apt/sources.list

Now change there, any word lenny to squeeze,(or from stable to testing) and save it, it could be a good idea to backup your original file before, just in case.

Updating and upgrading

sudo aptitude update

sudo aptitude install apt dpkg aptitude

sudo aptitude full-upgrade

And that is it, now you are running Debian Squeeze, it is better to use squeeze instead of testing, so when it change from testing to stable, you can decide when to go to testing again.