Upgrade Debian 11.7 to 12.0

Complete guide for Upgrading your Debian 11 Server or Desktop to Debian 12 (Bookworm).

Prerequisites

Backup your data (Critical!).
Ensure you have root access or sudo privileges.
Ensure you have a stable internet connection.

Step 1: Update Existing System

Before upgrading, make sure your current system is fully up to date.

sudo apt update
sudo apt upgrade -y
sudo apt full-upgrade -y
sudo apt autoremove -y

Step 2: Update Sources List

You need to change the package repositories from bullseye to bookworm.

Edit the sources list:
```
sudo nano /etc/apt/sources.list
```

Replace all instances of bullseye with bookworm. Also, non-free component has been split into non-free and non-free-firmware.

Example sources.list for Debian 12:

deb http://deb.debian.org/debian bookworm main contrib non-free non-free-firmware
deb http://deb.debian.org/debian bookworm-updates main contrib non-free non-free-firmware
deb http://security.debian.org/debian-security bookworm-security main contrib non-free non-free-firmware

Save and exit (Ctrl + O, Enter, Ctrl + X).

Step 3: Run the Upgrade

Update package lists with new sources:
```
sudo apt update
```
Perform a minimal upgrade first (to avoid conflicts):
```
sudo apt upgrade --without-new-pkgs -y
```
Perform the full distribution upgrade:
```
sudo apt full-upgrade -y
```
During this process:
- Several services will restart.
- You may be asked if you want to restart specifically. Choose Yes.
- You may be asked about configuration files. If you haven't customized them, choose "Install the package maintainer's version". If you have, choose "Keep your currently-installed version".

Step 4: Reboot and Verify

Restart your system:
```
sudo reboot
```

Verify the new version:

hostnamectl
# or
cat /etc/debian_version

Should show 12.x.

Step 5: Post-Upgrade Cleanup

Remove obsolete packages to free up space.

sudo apt --purge autoremove -y
sudo apt clean

Troubleshooting

"Firmware missing" errors

Debian 12 moved non-free firmware to a separate component. Ensure non-free-firmware is in your sources.list (as shown in Step 2).

SSH Disconnected

If upgrading a remote server, simple reboot is usually safe, but ensure you have console access (VNC/KVM) just in case network config changes break connectivity.