Ubuntu 默认的 apt 源下载速度不稳定，可以考虑切换到国内的 apt 镜像源。
Step1. 备份 list 文件
进入对应文件夹，用管理员权限，执行复制操作：
1cd /etc/apt/
2sudo cp sources.list sources.list.bak
Step2. 修改 list 文件
通过管理员权限，使用 vim 进行修改：
1sudo vim sources.list
将文件中所有 url 地址，修改为 http://mirrors.aliyun.com/ubuntu/ 即可。
Step3. 更新软件列表
使用管理员权限，执行命令：
1sudo apt-get update
2sudo apt-get upgrade
P.S.
命令apt-get update是更新软件列表
命令apt-get upgrade是更新软件
附：国内常见源
阿里云：
1deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
2deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
3deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
4deb http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
5deb http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
6deb-src http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
7deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
8deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
9deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
10deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
中科大：
1deb https://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse
2deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse
3deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
4deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
5deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
6deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
7deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
8deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
9deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
10deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
网易：
1deb http://mirrors.163.com/ubuntu/ bionic main restricted universe multiverse
2deb http://mirrors.163.com/ubuntu/ bionic-security main restricted universe multiverse
3deb http://mirrors.163.com/ubuntu/ bionic-updates main restricted universe multiverse
4deb http://mirrors.163.com/ubuntu/ bionic-proposed main restricted universe multiverse
5deb http://mirrors.163.com/ubuntu/ bionic-backports main restricted universe multiverse
6deb-src http://mirrors.163.com/ubuntu/ bionic main restricted universe multiverse
7deb-src http://mirrors.163.com/ubuntu/ bionic-security main restricted universe multiverse
8deb-src http://mirrors.163.com/ubuntu/ bionic-updates main restricted universe multiverse
9deb-src http://mirrors.163.com/ubuntu/ bionic-proposed main restricted universe multiverse
10deb-src http://mirrors.163.com/ubuntu/ bionic-backports main restricted universe multiverse
清华：
1deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricted universe multiverse
2deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricted universe multiverse
3deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
4deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
5deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
6deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
7deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
8deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
9deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
10deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
参考链接
Ubuntu18.04下更改apt源为阿里云源
Ubuntu 18.04换国内源 中科大源 阿里源 163源 清华源
ubuntu sudo update与upgrade的作用及区别
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