An operating system distribution often running a Linux Kernel
Upgrade Debian Jessie (8) to Debian Stretch (9)
apt-get update && apt-get -y upgrade && apt-get -y dist-upgrade && apt-get clean
restart if there were any updates like kernel and stuffs
sed -i "s/jessie/stretch/" /etc/apt/sources.list apt-get update && apt-get -u upgrade && apt-get -y dist-upgrade
apt-get update && apt-get -y upgrade
apt-get autoremove aptitude purge '~c'