1.备份原始源文件 sources.list

桌面打开终端，执行命令：

sudo  cp  /etc/apt/sources.list    /etc/apt/sources.list.bak

2.修改源文件sources.list

（1）终端执行命令：

sudo   chmod   777   /etc/apt/sources.list

更改文件权限使其可编辑；
（2）执行命令：

sudo   gedit   /etc/apt/sources.list

打开文件进行编辑；
（3）删除原来的文件内容，复制下面的任意一个到其中并保存（常用的是阿里源和清华源，推荐阿里源，内容可以去博客复制）；
　　阿里源：

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

3.更新源

桌面终端执行命令：

sudo apt update
sudo apt upgrade

更新软件列表，换源完成。