Debian 镜像配置文件位于/etc/apt/sources.list。
手动替换原有的/etc/apt/sources.list文件内容为：
# 默认注释了源码镜像以提高 apt update 速度，如有需要可自行取消注释
deb http://mirrors.pku.edu.cn/debian/ {debian_codename} main contrib non-free
# deb-src https://mirrors.pku.edu.cn/debian/ {debian_codename} main contrib non-free
deb http://mirrors.pku.edu.cn/debian/ {debian_codename}-updates main contrib non-free
# deb-src http://mirrors.pku.edu.cn/debian/ {debian_codename}-updates main contrib non-free
deb http://mirrors.pku.edu.cn/debian/ {debian_codename}-backports main contrib non-free
# deb-src http://mirrors.pku.edu.cn/debian/ {debian_codename}-backports main contrib non-free
deb http://mirrors.pku.edu.cn/debian-security {debian_codename}{debian_security} main contrib non-free
# deb-src http://mirrors.pku.edu.cn/debian-security {debian_codename}{debian_security} main contrib non-free
修改文件后需要更新索引：
sudo apt-get update