sed -i 's/buster/bullseye/g' /etc/apt/sources.list
find /etc/apt/sources.list.d -type f -exec sed -i 's/buster/bullseye/g' {} \;

sed -i 's|security.debian.org/debian-security buster/updates|security.debian.org/debian-security bullseye-security|' /etc/apt/sources.list

# One liner
apt-get update && sudo apt-get dist-upgrade --autoremove -y

# OR step by step
apt update
apt upgrade --without-new-pkgs
apt full-upgrade
apt --purge autoremove