# 先备份源列表文件
mv /etc/apt/sources.list /etc/apt/sources.list.bak
# 生成新的源列表文件(用的国内源镜像)
echo "deb http://ftp.cn.debian.org/debian/ stretch main" > /etc/apt/sources.list
echo "deb http://ftp.cn.debian.org/debian/ stretch-updates main" >> /etc/apt/sources.list
echo "deb http://ftp.cn.debian.org/debian-security stretch/updates main" >> /etc/apt/sources.list
# 更新源
apt update