This is a pretty painless and straight forward process.
Firstly, we want to make sure we are all up to date, so:
sudo bash apt-get update && apt-get upgrade && apt-get dist-upgrade
answer yes to any prompts.
Next we need to point apt at the Jessie repositories:
sed -i 's/wheezy/jessie/g' /etc/apt/sources.list
once we have done this, time to update again..
apt-get update && apt-get upgrade
You may be given an option to manually or automatically restart any currently running services. It is recommended to manually restart as it provides more flexibility, that said, I chose yes with no issues. (I had other things to do than watch a lengthy upgrade!) If you are running the SSH service you will be also given an option to disable SSH root access. If you opt to “YES” you will no longer be able to SSH login as root user, so make sure you read this and respond accordingly!
Finally, e get to do the upgrade, so one last time:
Depending what you have installed on your Pi, you may be prompted for MySQL Passwords etc during the upgrade. This may take a while, so go grab a refreshing beverage of your choice and maybe watch an episode of your favourite TV show.
Once complete, reboot the PI and enjoy Debian 8 (Jessie)