Update current System
Start with the update/upgrade of the current system to run the latest software packages prior the upgrade.
sudo apt-get update -y sudo apt-get upgrade -y sudo apt-get dist-upgrade
You can go on if the following command doesn't list any held packages:
sudo apt-mark showhold
Upgrade steps to Stretch
Switch Repo Definition:
sudo sed -i /deb/s/jessie/stretch/g /etc/apt/sources.list sudo sed -i /deb/s/jessie/stretch/g /etc/apt/sources.list.d/*.list
Refresh Repo Content:
sudo apt-get update -y
Perform Strech Upgrade
sudo apt-get upgrade sudo apt-get dist-upgrade
Go on and reboot your system to boot into Raspbian Stretch.