推荐升级路径，TL;DR：
备份数据，然后
apt update
apt upgrade
apt dist-upgrade
reboot
还要禁用所有非主流包（如果有）。
然后为新发行版准备 APT：
sudo sed -i 's/buster/bullseye/g' /etc/apt/sources.list
sudo sed -i 's/buster/bullseye/g' /etc/apt/sources.list.d/*
第一个命令显示错误：
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.