sed -i 's/bullseye/bookworm/g' /etc/apt/sources.list
sed -i 's/bullseye/bookworm/g' /etc/apt/sources.list.d/pgdg.list
sed -i 's/debian 11/debian 12/g' /etc/apt/sources.list.d/zammad.list
apt clean; apt update
apt upgrade
apt dist-upgrade

pg_dropcluster --stop 13 main
pg_upgradecluster 13 main /var/lib/postgresql/15/

apt autoremove
shutdown -r now