sed -i 's/bionic/focal/g' /etc/apt/sources.list
sed -i 's/ubuntu 18/ubuntu 20/g' /etc/apt/sources.list.d/zammad.list
apt clean; apt update
apt upgrade
apt dist-upgrade

pg_dropcluster --stop 13 main
pg_upgradecluster 11 main /var/lib/postgresql/13/

apt autoremove
shutdown -r now