Update Debian 10 (Buster) to Debian 11 (Bullseye)

Prepare:

apt update
apt upgrade -qqy
apt dist-upgrade -qqy

Update APT:

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

Upgrade:

apt update && \
apt upgrade -qqy --without-new-pkgs && \
apt full-upgrade -qqy && \
apt --purge autoremove -yqq

Reboot:

reboot