Buster

Important This fix can be applied before the reboot after the upgrade from Buster to Bullseye, if you missed it you need a MicroSD card reader and some Linux computer (another RaspberryPI would do) to edit the file. In the file /etc/systemd/system/dhcpcd.service.d/wait.conf there is a wrong path for the dhcpd, it has to be changed. wrong (from buster): /usr/lib/dhcpcd5/dhcpcd correct and new on Bullseye: /usr/sbin/dhcpcd So you need to do this before the reboot, otherwise the RaspberryPi will not have a working WIFI or Ethernet connection after the reboot and will not allow you to login from remote by SSH. ...

So to upgrade Raspbian, as it was previously called, from Buster to Bullseye went smooth an easy, I tried this with some Pi2B (armhf 32Bit) and with a Pi4. These two are headless systems, so I don’t know if this works well for any systems with window managers. Better run these commands in screen, so when the SSH connection is interrupted you can reconnect and resume your work. Edit: For the update there is a culprit, it will cause a problem with dhcpd, so before the reboot 13. (was point 12. before I added the remark below) please check this post. ...