sudo sed -i 's/buster/bullseye/g' /etc/apt/sources.list
sudo sed -i 's/buster/bullseye/g' /etc/apt/sources.list.d/*
sudo sed -i 's/bullseye\/updates/bullseye-security/g' /etc/apt/sources.list

sudo apt update
sudo apt upgrade --without-new-pkgs
sudo apt full-upgrade
reboot

sudo apt purge linux-image-4.19*