In this post I am going to explain how to upgrade Debian 8 to Debian 9. You can enter the following eight steps in your terminal:

1. sudo apt-get update
2. sudo apt-get upgrade
3. sudo apt-get dist-upgrade
4. sed -i ‘s/jessie/stretch/g’ /etc/apt/sources.list
5. sudo apt-get update
6. sudo apt-get upgrade
7. sudo apt-get autoremove
8. reboot