see also:
https://www.elektronik-kompendium.de/sites/raspberry-pi/2204031.htm

-----------------

steps:

1) find out installed version:
cat /etc/issue
>>Raspbian GNU/Linux 8 \n \l

>> find out the names: https://en.wikipedia.org/wiki/Raspberry_Pi_OS
Debian 8 = Jessie
Debian 9 = Stretch
Debian 10 = Buster

>> we have Raspbian 8 >> let's upgrade to Raspbian 9
>> Jessie > Stretch


2) install latest packages:
sudo apt-get update
sudo apt-get dist-upgrade

3) package respositories
/etc/apt/sources.list
>> deb http://mirrordirector.raspbian.org/raspbian/ stretch main contrib non-free rpi
(comment out the other lines!)


/etc/apt/sources.list.d/raspi.list
>> deb http://archive.raspberrypi.org/debian/ stretch main ui
(comment out the other lines!)

4) get new packages
sudo apt-get update

5) do the release upgrade
sudo apt-get upgrade
sudo apt-get dist-upgrade

6) clean up after upgrade
sudo apt-get autoremove
sudo apt-get autoclean

7) restart of the machine
sudo reboot

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