After release of Debian 6.0 (squeeze) the question of how to upgrade from previous version to the latest.
It appears to be fairly simple process:
1. Edit sources.list
2. Replace distribution name (I appeared to have Lenny) with Squeeze.
Note: DebianVolatile has been superseded by squeeze-updates since Debian Squeeze.
To use squeeze-updates just add
deb http://mirrors.kernel.org/debian squeeze-updates main contrib
Then as usual: