Q: How to upgrade a Debian system from stable to testing (or to unstable)?
A: A normal installation process installs a Debian "stable" system.
You can upgrade from "stable" to "testing" by changing the distribution
suite name string in the file "/etc/apt/sources.list" from "stable" to
"testing" (or by changing the distribution codename string from
"&releasename;" to "&testingreleasename;").
You can upgrade from "stable" to "unstable" by changing the distribution
suite name string in the file "/etc/apt/sources.list" from "stable" to
"unstable" (or by changing the distribution codename string from
"&releasename;" to "sid").
After this change, a regular upgrade (e.g. by running "aptitude dist-upgrade")
will execute the full upgrade.
These operations come with some risks. Please read "Life with eternal
upgrades" in "Debian Reference" for details.