Debian OS version Upgrade from buster to bullseye

Step 1:

cp -pv /etc/apt/sources.list /etc/apt/sources.list.bak

Step 2, replacing OS name in the sources.list file:

nano /etc/apt/sources.list

Before:

deb http://deb.debian.org/debian/ buster main
deb-src http://deb.debian.org/debian/ buster main
deb http://deb.debian.org/debian/ buster-updates main
deb-src http://deb.debian.org/debian/ buster-updates main
deb http://security.debian.org/debian-security buster/updates main
deb-src http://security.debian.org/debian-security buster/updates main

After:
deb http://deb.debian.org/debian/ bullseye main
deb-src http://deb.debian.org/debian/ bullseye main
deb http://deb.debian.org/debian/ bullseye-updates main
deb-src http://deb.debian.org/debian/ bullseye-updates main
deb http://security.debian.org/debian-security bullseye/updates main
deb-src http://security.debian.org/debian-security bullseye/updates main
and an additional command to resolve the possible issue with one of the repository:
sed -i 's/bullseye\/updates/bullseye-security/g' /etc/apt/sources.list

Step 3, upgrade packages:
apt-get update
apt-get upgrade

Step 4, upgrade OS:
apt-get dist-upgrade
reboot

Step 5, final check:
hostnamectl

Very good post that used by me before.