Update Debian 8 (Jessie) to Debian 9 (Stretch)

Prepare:

apt update
apt upgrade -qqy
apt dist-upgrade -qqy

Update APT:

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

Upgrade:

apt update && \
apt upgrade -qqy && \
apt dist-upgrade -qqy && \
apt --purge autoremove -yqq

Reboot:

reboot