Since few hours, Debian 9 Stretch is released to stable version. So, let’s upgrade our Debian 8 Jessie system to Debian 9 Stretch in few commands.

First, we upgrade Debian 8 Jessie system:

apt-get update
apt-get upgrade
apt-get dist-upgrade

Then, update sources file (/etc/apt/sources.list), like this:

#From Debian 8 Jessie
deb http://httpredir.debian.org/debian jessie main
deb http://httpredir.debian.org/debian jessie-updates main
deb http://security.debian.org jessie/updates main
#From Debian 9 Stretch
deb http://httpredir.debian.org/debian stretch main
deb http://httpredir.debian.org/debian stretch-updates main
deb http://security.debian.org stretch/updates main

Upgrade the index package with this command:

apt-get update

Finally, proceed to the Debian 9 Stretch upgrade

apt-get upgrade
apt-get dist-upgrade