Raspbian

Upgrade from Buster 10 to Bullseye 11

sudo apt update
sudo apt dist-upgrade -y

# update firmware
sudo rpi-update

sudo nano /etc/apt/sources.list
# deb http://raspbian.raspberrypi.org/raspbian/ buster main contrib non-free rpi
deb http://raspbian.raspberrypi.org/raspbian/ bullseye main contrib non-free rpi

sudo apt update
sudo apt dist-upgrade
sudo apt autoclean
sudo reboot

ref: [1]