Doing recommended upgrade path, TL;DR:
Backup data, then
apt update
apt upgrade
apt dist-upgrade
reboot
Also disable all non main stream packages if any.
Then preparing APT for new distribution:
sudo sed -i 's/buster/bullseye/g' /etc/apt/sources.list
sudo sed -i 's/buster/bullseye/g' /etc/apt/sources.list.d/*
The first command shows error:
apt update
...
Err:6 http://security.debian.org/debian-security bullseye/updates Release
404 Not Found
Reading package lists... Done
E: The repository 'http://security.debian.org/debian-security bullseye/updates Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
Best Answer
This is regarding change in Debian 11 repository structure. RTFM requires time, so bump answer here:
Thus execute one more command:
Now apt update works great again, and you can continue with upgrade:
Check if system is upgraded:
After successful reboot, you can remove old kernel:
Also review log files, for error messages of conflicting configurations or other issues.
Issue list is here: https://www.debian.org/releases/bullseye/amd64/release-notes/ch-information.html