tl;dr

To upgrade (K)ubuntu from 19.04 to 20.04, perform the following steps:

# Attention: de.archive is for the german version. You may have to adapt it
# First step: Update everything from the current version
sudo sed -i 's/de.archive/old-releases/g' /etc/apt/sources.list
sudo apt-get update -y
sudo apt-get upgrade -y
sudo apt-get dist-upgrade -y
# Second step: Do the upgrade
sudo sed -i 's/disco/focal/g' /etc/apt/sources.list
sudo sed -i 's/old-releases/de.archive/g' /etc/apt/sources.list
sudo apt-get update -y
sudo apt-get upgrade -y
sudo apt-get dist-upgrade -y

Now reboot and make sure you have the correct version:

cat /etc/os-release

Long version

Recently I wanted to install the brave browser to my Kubuntu system, but I observed that I was not able to install any packages due to 404 errors during apt-get install command:

Err http://archive.ubuntu.com main/restricted i386 Packages
  404  Not Found [IP: 91.189.92.200 80]
Ign http://ppa.launchpad.net trusty/main Translation-en_GB
Err http://archive.ubuntu.com main/multiverse i386 Packages
  404  Not Found [IP: 91.189.92.200 80]
  ...

After a while I found out, that my system was too old. I was running a Kubuntu 19.04 but the latest version was 20.04, so I decided to upgrade my system. To do so I had to update all packages first. As this was not possible via apt-get, the first step was to replace all occurrences in the /etc/apt/sources.list file of archive.ubuntu by old-releases.ubuntu. This foreces apt to fetch packages from the archived packages repositories rather than from the normal ones. You can achieve this by using sed command like so:

sudo sed -i 's/de.archive/old-releases/g' /etc/apt/sources.list

After this has been done, I was able to update via apt-get update and apt-get dist-upgrade to newer packages.

After that I tried to upgrade my Kubuntu. To do so I followed the steps described in this brilliant article. Unfortunately it was not possible for me to update a disco system to focal system directly, so I had to find another way. In order to update the system, I had to revert the previously done changes and edit the sources.list file again. This time I had to replace the old-releases occurrences by archive and in addition to that, I had to replace all occurrences of disco by focal.

sudo sed -i 's/disco/focal/g' /etc/apt/sources.list
sudo sed -i 's/old-releases/de.archive/g' /etc/apt/sources.list

Finally I was able to do an apt-get update and an apt-get dist-upgrade. After a reboot I noticed that I now was running Kubuntu focal by running

cat /etc/os-release