Upgrade from Debian-7 Wheezy to Debian-8 Jessie

Lets look how to upgrade Debian Linux from Wheezy to Jessie stable

Fully Upgrade current System
apt-get update
apt-get upgrade
apt-get dist-upgrade

Upgrade to Debian Jessie
sed -i 's/wheezy/jessie/g' /etc/apt/sources.list
apt-get update
apt-get upgrade

During the upgrade you may be given an option to manually or automatically restart any currently running services. The recommended strategy is to do manual restart as it provides more flexibility. If you are running SSH service you will be also given an option to disable SSH root access. What it means, that if you opt to "YES" you will no longer be able to SSH login as root user.

If there are no issues with the package upgrade we are ready to perform a full system distribution upgrade from Debian Wheezy to Debian Jessie.
apt-get dist-upgrade

After the upgrade it is recommended to reboot your system:
reboot

Check your upgrade:
hostnamectl