https://www.tomshardware.com/how-to/upgrade-raspberry-pi-os-to-bullseye-from-buster
A new Raspberry Pi OS based upon Debian 11 (Codenamed: Bullseye, following a Toy Story character naming convention) has been dropped and it has Raspberry Pi fans clamouring to install the latest OS on their favorite single board computer.
There are big changes for the latest Raspberry Pi OS, including a free speed boost for Raspberry Pi 4 / Compute Module 4 owners who have a certain SoC model, and a new compositing window manager which speeds up overall desktop performance.
Raspberry Pi OS “Bullseye” works with all models of Raspberry Pi, but the best performance is found using Raspberry Pi 4, Compute Module 4 and Pi 400. These models can benefit from the speed boost (except the Pi 400, where it is the default). If they have 2GB of RAM or greater, they can also use the new compositing window manager.
The ideal way to install the latest OS is to backup all your project and data files and then do a clean install using Raspberry Pi Imager to write to a microSD card, but it is possible to upgrade an old Debian 10 “Buster” based Raspberry Pi OS to Bullseye.There is a caveat, however, as it may not be the smoothest of upgrades. So before you take the plunge make sure that you have a backup of any important files and projects.
How to Upgrade to the Latest Raspberry Pi OS
1. Open a terminal and update the software repositories lists.
$ sudo apt update
2. Run a full upgrade of your installation.
$ sudo apt dist-upgrade -y
3. Update the Raspberry Pi firmware.
$ sudo rpi-update
4. Open the sources.list file ready for editing.
$ sudo nano /etc/apt/sources.list
(Optional) $ sudo nano /etc/apt/sources.list.d/raspi.list
5. Using the arrow keys, scroll down the file and locate the following line.
deb http://raspbian.raspberrypi.org/raspbian/ buster main contrib non-free rpi
6. Change the line from buster to bullseye and press CTRL + X, then press Y and Enter to save and exit.
deb http://raspbian.raspberrypi.org/raspbian/ bullseye main contrib non-free rpi
7. Update the Bullseye software repositories.
$ sudo apt update
8. Run another distribution upgrade to install the Bullseye software requirements.
$ sudo apt dist-upgrade
9. Clean up the installation files.
$ sudo apt autoclean
10. Reboot your Raspberry Pi for the change to take effect.
$ sudo reboot
You are now running the latest version of Raspberry Pi OS based upon “Bullseye”.
