默认更新源速度较慢，可以通过修改系统默认下载源为国内镜像源加速下载过程。

Step1 备份当前下载源文件

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

Step2 修改sources.list

sudo vim sources.list

清空该文件，选择国内镜像源粘贴进去，以阿里源为例：

# deb包
deb http://mirrors.aliyun.com/ubuntu/ xenial main restricted universe multiverse  
deb http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted universe multiverse  
deb http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted universe multiverse  
deb http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse  
# 测试版源  
deb http://mirrors.aliyun.com/ubuntu/ xenial-proposed main restricted universe multiverse  

##  以下为源码，建议注释掉，不然后期使用 apt-update 更新时可能会引发多次编译的错误
# 源码  
# deb-src http://mirrors.aliyun.com/ubuntu/ xenial main restricted universe multiverse  
# deb-src http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted universe multiverse  
# deb-src http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted universe multiverse  
# deb-src http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse  
# 测试版源  
# deb-src http://mirrors.aliyun.com/ubuntu/ xenial-proposed main restricted universe multiverse  
# Canonical 合作伙伴和附加  
deb http://archive.canonical.com/ubuntu/ xenial partner  
deb http://extras.ubuntu.com/ubuntu/ xenial main

Step3 保存，退出，更新

sudo apt-get update
sudo apt-get upgrade

Raspberry Pi

使用管理员权限（经由sudo），编辑 /etc/apt/sources.list 文件。参考命令行为：

$ sudo vim /etc/apt/sources.list

用 # 注释掉原文件内容，用以下内容取代：

deb http://mirrors.tuna.tsinghua.edu.cn/raspbian/raspbian/ stretch main contrib non-free rpi
deb-src http://mirrors.tuna.tsinghua.edu.cn/raspbian/raspbian/ stretch main contrib non-free rpi

使用管理员权限（经由sudo），编辑 /etc/apt/sources.list.d/raspi.list 文件。参考命令行为：

$ sudo vim /etc/apt/sources.list.d/raspi.list

用 # 注释掉原文件内容，用以下内容取代：

deb http://mirror.tuna.tsinghua.edu.cn/raspberrypi/ stretch main ui
deb-src http://mirror.tuna.tsinghua.edu.cn/raspberrypi/ stretch main ui