方法一、通过图形化界面跟换软件源
进入设置：点击software&updates
选择download from，点击展开，选择others
通过手动查找列表或者切换至推荐服务器
通过自动检测，将软件源切换到ustc
进入/etc/apt/sources.list查看软件源：
nano /etc/apt/sources.list
软件源已经切换到ustc
方法二、
检验软件源：
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
nano /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：
debhttp://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
debhttp://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universemultiverse
debhttp://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universemultiverse
debhttp://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universemultiverse
debhttp://mirrors.aliyun.com/ubuntu/ bionic-backports main restricteduniverse multiverse
deb-srchttp://mirrors.aliyun.com/ubuntu/ bionic main restricted universemultiverse
deb-srchttp://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universemultiverse
deb-srchttp://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universemultiverse
deb-srchttp://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universemultiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backportsmain restricted universe multiverse
清华源：
debhttps://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricteduniverse multiverse
deb-srchttps://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricteduniverse multiverse
debhttps://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates mainrestricted universe multiverse
deb-srchttps://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates mainrestricted universe multiverse
debhttps://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports mainrestricted universe multiverse
deb-srchttps://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports mainrestricted universe multiverse
debhttps://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security mainrestricted universe multiverse
deb-srchttps://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security mainrestricted universe multiverse
debhttps://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed mainrestricted universe multiverse
deb-srchttps://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed mainrestricted universe multiverse
163源：
debhttp://mirrors.163.com/ubuntu/ bionic main restricted universe multiverse
debhttp://mirrors.163.com/ubuntu/ bionic-security main restricted universemultiverse
deb http://mirrors.163.com/ubuntu/ bionic-updatesmain restricted universe multiverse
debhttp://mirrors.163.com/ubuntu/ bionic-proposed main restricted universemultiverse
debhttp://mirrors.163.com/ubuntu/ bionic-backports main restricted universemultiverse
deb-src http://mirrors.163.com/ubuntu/ bionicmain restricted universe multiverse
deb-srchttp://mirrors.163.com/ubuntu/ bionic-security main restricted universemultiverse
deb-srchttp://mirrors.163.com/ubuntu/ bionic-updates main restricted universemultiverse
deb-srchttp://mirrors.163.com/ubuntu/ bionic-proposed main restricted universemultiverse
deb-srchttp://mirrors.163.com/ubuntu/ bionic-backports main restricted universemultiverse
3.更新源
桌面终端执行命令：sudo apt update更新软件列表，换源完成。