考虑到debian源的访问有效性和速度问题，比如国外VPS用默认的debian官方源可能没啥问题，但是国内vps由于种种原因可能无法访问官方源或者速度非常慢，就需要我们在系统配置文件里更新软件源，以下为更新方法，留存备用：

1、首先备份配置文件（万一搞砸了还能还原回去）

mv /etc/apt/sources.list /etc/apt/sources.list.old

2、更新源为离你较为近的一个（选一个即可）：

Debian阿里云源：

cat > /etc/apt/sources.list << EOF
deb https://mirrors.aliyun.com/debian/ bullseye main contrib non-free
deb-src https://mirrors.aliyun.com/debian/ bullseye main contrib non-free
deb https://mirrors.aliyun.com/debian/ bullseye-updates main contrib non-free
deb-src https://mirrors.aliyun.com/debian/ bullseye-updates main contrib non-free
deb https://mirrors.aliyun.com/debian/ bullseye-backports main contrib non-free
deb-src https://mirrors.aliyun.com/debian/ bullseye-backports main contrib non-free
deb https://mirrors.aliyun.com/debian-security/ bullseye-security main contrib non-free
deb-src https://mirrors.aliyun.com/debian-security/ bullseye-security main contrib non-free
EOF

Debian腾讯云源：

cat > /etc/apt/sources.list << EOF
deb http://mirrors.tencentyun.com/debian/ bullseye main contrib non-free
deb-src http://mirrors.tencentyun.com/debian/ bullseye main contrib non-free
deb http://mirrors.tencentyun.com/debian/ bullseye-updates main contrib non-free
deb-src http://mirrors.tencentyun.com/debian/ bullseye-updates main contrib non-free
deb http://mirrors.tencentyun.com/debian/ bullseye-backports main contrib non-free
deb-src http://mirrors.tencentyun.com/debian/ bullseye-backports main contrib non-free
deb http://mirrors.tencentyun.com/debian-security/ bullseye-security main contrib non-free
deb-src http://mirrors.tencentyun.com/debian-security/ bullseye-security main contrib non-free
EOF

Debian Linode源：

cat > /etc/apt/sources.list << EOF
deb http://mirrors.linode.com/debian/ bullseye main contrib non-free
deb-src http://mirrors.linode.com/debian/ bullseye main contrib non-free
deb http://mirrors.linode.com/debian/ bullseye-updates main contrib non-free
deb-src http://mirrors.linode.com/debian/ bullseye-updates main contrib non-free
deb http://mirrors.linode.com/debian/ bullseye-backports main contrib non-free
deb-src http://mirrors.linode.com/debian/ bullseye-backports main contrib non-free
deb http://mirrors.linode.com/debian-security/ bullseye-security main contrib non-free
deb-src http://mirrors.linode.com/debian-security/ bullseye-security main contrib non-free
EOF

Debian 官方源：

cat > /etc/apt/sources.list << EOF
deb https://deb.debian.org/debian/ bullseye main contrib non-free
deb-src https://deb.debian.org/debian/ bullseye main contrib non-free
deb https://deb.debian.org/debian/ bullseye-updates main contrib non-free
deb-src https://deb.debian.org/debian/ bullseye-updates main contrib non-free
deb https://deb.debian.org/debian/ bullseye-backports main contrib non-free
deb-src https://deb.debian.org/debian/ bullseye-backports main contrib non-free
deb https://deb.debian.org/debian-security/ bullseye-security main contrib non-free
deb-src https://deb.debian.org/debian-security/ bullseye-security main contrib non-free
EOF

Debian 清华源：

cat > /etc/apt/sources.list << EOF
deb https://mirrors.tuna.tsinghua.edu.cn/debian/ bullseye main contrib non-free
deb-src https://mirrors.tuna.tsinghua.edu.cn/debian/ bullseye main contrib non-free
deb https://mirrors.tuna.tsinghua.edu.cn/debian/ bullseye-updates main contrib non-free
deb-src https://mirrors.tuna.tsinghua.edu.cn/debian/ bullseye-updates main contrib non-free
deb https://mirrors.tuna.tsinghua.edu.cn/debian/ bullseye-backports main contrib non-free
deb-src https://mirrors.tuna.tsinghua.edu.cn/debian/ bullseye-backports main contrib non-free
deb https://mirrors.tuna.tsinghua.edu.cn/debian-security/ bullseye-security main contrib non-free
deb-src https://mirrors.tuna.tsinghua.edu.cn/debian-security/ bullseye-security main contrib non-free
EOF

Debian 中科大源：

cat > /etc/apt/sources.list << EOF
deb https://mirrors.ustc.edu.cn/debian/ bullseye main contrib non-free
deb-src https://mirrors.ustc.edu.cn/debian/ bullseye main contrib non-free
deb https://mirrors.ustc.edu.cn/debian/ bullseye-updates main contrib non-free
deb-src https://mirrors.ustc.edu.cn/debian/ bullseye-updates main contrib non-free
deb https://mirrors.ustc.edu.cn/debian/ bullseye-backports main contrib non-free
deb-src https://mirrors.ustc.edu.cn/debian/ bullseye-backports main contrib non-free
deb https://mirrors.ustc.edu.cn/debian-security/ bullseye-security main contrib non-free
deb-src https://mirrors.ustc.edu.cn/debian-security/ bullseye-security main contrib non-free
EOF

3、执行完第2步操作后，再更新软件即可：

apt update && apt upgrade