换成国内最快的阿里云源

第一步：备份原来的源文件

cd /etc/apt/

然后会显示下面的源文件sources.list
输入命令
sudo cp sources.list sources.list.bak
就是将sources.list备份到sources.list.bak

第二步：替换源

阿里云源的文件

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  
    # 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

替换并保存
sudo gedit sources.list打开文件，替换成阿里云文件即可

第三步：更新源和软件

sudo apt-get update 更新源  
   sudo apt-get upgrade 更新软件