One of Debian's goals is to provide a smooth, secure and reliable upgrade
process. The packaging system alerts the administrator to important changes
and sometimes asks the administrator to take decisions. You should also read
the Release Notes; it is shipped on all Debian CDs and is available on the WWW
at https://www.debian.org/releases/stable/releasenotes
or https://www.debian.org/releases/testing/releasenotes.
A practical guide to upgrades is provided in Debian package management, Chapter 6. This
section merely provides an outline, beginning with the packaging tools.