How to upgrade to Debian Linux 9 (stretch)

1. Update all packages currently installed to the latest versions

apt-get update
apt-get upgrade


2. Replace all occurrences of jessie with stretch in the apt sources file /etc/apt/sources.list using your favorite text editor (ex. pico /etc/apt/sources.list).

Here’s an example of what the file could look like after the change:

deb http://ftp.debian.org/debian/ stretch main
deb http://security.debian.org/ stretch/updates main


Alternatively you can use the sed command to replace the reference to the repository:

sed -i 's/jessie/stretch/g' /etc/apt/sources.list


3. Update the apt package index

apt-get update


4. Upgrade all packages

apt-get upgrade
apt-get dist-upgrade


5. Once all packages are upgraded, restart the system

reboot


Your system has now been upgraded to Debian Linux 9 (Stretch)