Raspberry released a new major version for Raspberry Pi's yesterday 8.11.2021. You can check their article here. I went to upgrade the system immediately. There was no news article on how to update from Buster (Debian 10) to Bullseye (Debian 11) so I did it myself, noting that I've never updated a Linux system.
The first thing I did was update packages just in case.
sudo apt update
Then I went to the /etc/apt/sources.list file. Changed "buster" to "bullseye"
deb http://raspbian.raspberrypi.org/raspbian/ bullseye main contrib non-free rpi
Then I ran the following scripts:
sudo apt update sudo apt dist-upgrade
Then I rebooted my server
If you have some services that need to be started on system startup, use Systemd. We use it too! We did a status update when we upgraded the system. It went pretty fast and without any issues, except we lost outside network connection to MariaDB, but we fixed it quickly.