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Upgrade Debian 10 Buster to Debian 11 Bullseye

29 October, 2026
Linux

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How to upgrade from Debian 10 Buster to Debian 11 Bullseye

In this guide, we've got a server that's running Debian 10 that needs to be upgraded to Debian 12.
Direct upgrades from Buster to Bookworm are not supported so we first have to upgrade from Debian 10 to 11, and then to 12. 
Phew, lets get started! 

1. Getting ready for the upgrade

Backup

Before we get started, we'll take a full backup of the server

Check disk space

And we'll check there is enough free disk space to do the upgrade

df -h

 Check Debian OS version

# check os verison
cat /etc/*-release
cat /etc/debian_version

check debian version 

Check third party repositories

Any third party repositories will also need to be updated for Debian 11 sources and the applications upgraded

ls /etc/apt/sources.list.d/

mariadb nginx and php are installed so they will have to be updated to Bullseye as part of the main OS upgrade
We're not going to cover the steps for these individually as this will be different depending on what applications you've installed

third party repos

Install updates and reboot

Refresh apt sources, install updates and restart

# install updates
apt update
apt upgrade

# restart
systemctl reboot

2 Edit apt sources

Edit apt sources and change to Debian 11 Bullseye

nano /etc/apt/sources.list

apt sources.list before changes

# main
deb http://deb.debian.org/debian/ buster main contrib non-free
deb-src http://deb.debian.org/debian/ buster main contrib non-free

# security
deb http://security.debian.org/debian-security buster/updates main contrib non-free
deb-src http://security.debian.org/debian-security buster/updates main contrib non-free

# buster-updates, previously known as 'volatile'
deb http://deb.debian.org/debian/ buster-updates main contrib
deb-src http://deb.debian.org/debian/ buster-updates main contrib

# buster-backports
deb http://deb.debian.org/debian buster-backports main

 

main
replace buster with bullseye

# main
deb http://archive.debian.org/debian bullseye main contrib non-free
deb-src http://archive.debian.org/debian bullseye main contrib non-free

security
replace buster/updates with bullseye-security

# security
deb http://archive.debian.org/debian-security bullseye-security main contrib non-free
deb-src http://archive.debian.org/debian-security bullseye-security main contrib non-free

updates
replace buster-updates with bullseye-updates

# bullseye-updates, previously known as 'volatile'
deb http://archive.debian.org/debian bullseye-updates main contrib
deb-src http://archive.debian.org/debian bullseye-updates main contrib

backports
replace buster-backports with bullseye-backports

# bullseye-backports
deb http://archive.debian.org/debian bullseye-backports main

New apt sources.list for Bullseye


# main
deb http://archive.debian.org/debian bullseye main contrib non-free
deb-src http://archive.debian.org/debian bullseye main contrib non-free

# security
deb http://archive.debian.org/debian-security bullseye-security main contrib non-free
deb-src http://archive.debian.org/debian-security bullseye-security main contrib non-free

# bullseye-updates, previously known as 'volatile'
deb http://archive.debian.org/debian bullseye-updates main contrib
deb-src http://archive.debian.org/debian bullseye-updates main contrib

# bullseye-backports
deb http://archive.debian.org/debian bullseye-backports main

 apt sources list

 3 Update third party repos

Edit apt sources for third party repositories for MariaDB, Nginx and PHP

ls /etc/apt/sources.list.d/
mariadb.sources  nginx.list  php.list

We're not covering these step by step in this guide because each application will be different

4 Run minimal upgrade

Run minimal upgrade first before running the full OS upgrade

# refresh apt packages
apt update

# run minimal upgrade
apt upgrade --without-new-pkgs

apt update 

minimal upgrade

You will get prompted for any package changes that need you to confirm or changes to config files

Installation finished. No error reported.

minimal upgrade finished

 Restart

systemctl reboot

5 Run full upgrade

Run the full OS upgrade

apt full-upgrade

 full upgrade

 Restart

# reboot
systemctl reboot

Check OS version

cat /etc/debian_version
cat /etc/*-release

debian upgraded successfully2