由于默认的apt源下载很慢，所以我们需要修改成国内源：
1.首先备份之前的源：
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak
2.然后打开源文件
:
sudo vim /etc/apt/sources.list
3.我们这里要修改成阿里的源：
deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
4.然后更新软件列表：
sudo apt-get update
就变成阿里的源了，网速也快了不少！！！ 更新软件包
5.更新软件包：
sudo apt-get upgrade
