Update Raspbian 8 Jessie to 9 Stretch

sudo apt-get -y update 
sudo apt-get -y upgrade 
sudo apt-get -y dist-upgrade 
sudo dpkg -C
sudo apt-mark showhold

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

sudo apt-get -y update
sudo apt-get -y upgrade 
sudo apt-get -y dist-upgrade

reboot

find the debian version : (8 = jessie, 9 =stretch)

cat /etc/debian_version
cat /etc/os-release

sudo apt-get -y autoremove
sudo apt-get -y autoclean
sudo apt-get -y clean

Completed.....