升级Ubuntu系统时遇到的这个问题， 原因是默认的源不是国内的，而且还可能导致下载慢等问题。
1.备份源
cd /etc/apt/
sudo cp sources.list sources.list.bat
2.更换源
sudo vim sources.list
清除sources.list内的所有内容.更换为
#阿里云 Ubuntu 18.04
deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
按“ [Shift] +ZZ ”保存并退出
3.刷新
apt-get update
apt-get upgrade
OK 啦 ！
温馨提示：
更新系统后要换新的源->hugboy源库