I have found simple procedure on The Internet, which describes The upgrading from Lenny to Squeeze. What do You think about this procedure?
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
I Am triing this procedure and i will let You know The results.