1. Edit sources.list

sudo vi /etc/apt/sources.list

and change all words etch to lenny, or stable to testing (vim: :s/etch/lenny/g)

2. Update and upgrade

sudo aptitude update
sudo aptitude install apt dpkg aptitude
sudo aptitude full-upgrade

that is all, this also work if you want to go from lenny to sid, just change your sources.list accordingly.

It is also better if you start your Debian Linux in text mode, so GDM or KDM could be restarted without problems.

After the upgrade:

• If you run bastille, see here to fix a problem that may come up: http://www.epr.ch/brb/wiki/blog/running_bastille_on_debian_lenny

Ref: http://www.go2linux.org/upgrading-debian

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