換源准備：
換源之前明確使用平台，X86架構、ARM架構的源，ubuntu18.04和unbuntu16.04等源均不同，切忌病急亂投醫，看到一個源復制過來添加后沒用。
分析一條源：
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu/bionic-updates main restricted universe multiverse
從左到又依次為：
鏡像網址:http://mirrors.tuna.tsinghua.edu.cn
計算機架構對應關系：
| 計算機架構 | 代號 |
| X86 | ubuntu |
| ARM | ubuntu-ports |
系統版本號對應關系:
| 版本 | 版本號 | 代號 |
| Lucid(10.04) | 10.04 | lucid |
| Precise(12.04) | 12.04 | precise |
| Trusty(14.04) | 14.04 | trusty |
| Utopic(14.10) | 14.10 | utopic |
| Ubuntu 16.04 TLS | 16.04 | xenial |
| Ubuntu 18.04 TLS | 18.04 | bionic |
綜上TX2（18.04系統版本）的一條更新格式為
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-updates main restricted universe multiverse
換源步驟
1.獲得root權限
sudo su
2.打開源配置文件
gedit /etc/apt/sources.list
3.刪除原來的文件並且換成自己想換的源
#清華的源(ARM)
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ xenial-updates main restricted universe multiverse
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ xenial-updates main restricted universe multiverse
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ xenial-security main restricted universe multiverse
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ xenial-security main restricted universe multiverse
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ xenial-backports main restricted universe multiverse
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ xenial-backports main restricted universe multiverse
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ xenial main universe restricted
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ xenial main universe restricted
#阿里源(x86)
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
4.更新軟件列表
apt update
5.更新軟件包
apt upgrade
附組件：
main： 免費，Ubuntu 官方維護的自由軟件； restricted： 不免費，但是正規支持的，一般是設備專有驅動； universe：免費，社區維護的自由軟件； multiverse：不免費，無正規支持，非自由軟件；
security：重要的安全更新；
updates：非安全性更新，即不影響到系統安全的 bug 修補；
proposed：預更新，小 beta 版，過后會進入[updates]或[security]；
backports：新軟件的反向導出，新軟件為老系統編譯的版本；
組件參考：http://www.voidcn.com/article/p-zctkgwyc-bsd.html
| 版本 |
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