Upgrade debian 8 to 9

Update and upgrade first

apt-get update
apt-get upgrade

Remove the unused

apt-get autoremove

Run dist upgrade

apt-get dist-upgrade

Check current release information

lsb_release -a

Review the sources.list

cat /etc/apt/sources.list

Modify the sources.list to debian 9 source

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

Review it again

cat /etc/apt/sources.list

Update it

apt-get update

Check how many space should be used

apt-get -o APT::Get::Trivial-Only=true dist-upgrade

Check current space

df -H

All fine, then upgrade it

apt-get upgrade

Run the dist upgrade

apt-get dist-upgrade

Check the latest version

lsb_release -a

Check the kernel version

uname -mrs

Finllay, reboot it (not required), and check the kernel again

shutdown -r now
uname -mrs