Unable to locate package

                                  Unable to locate package
     在进行ftp和web服务器安装时采用apt-get方式安装一些软件，却总是报出这个错误：Unable to locate package，试了许多办法，改了许多配置文件就是没用，最后才知道是要更新update和upgrade。
     1.sudo apt-get install update 
        sudo apt-get install upgrade
	 2.但是upgrade时又报错了，就是现实不能安装。
	 3.上网查了，是要换成阿里云镜像。
	 4.阿里云镜像如下：
	 # deb cdrom:[Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1)]/ xenial main restricted deb-src http://archive.ubuntu.com/ubuntu xenial main restricted #Added by software-properties deb http://mirrors.aliyun.com/ubuntu/ xenial main restricted deb-src http://mirrors.aliyun.com/ubuntu/ xenial main restricted multiverse universe #Added by software-properties deb http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted deb-src http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted multiverse universe #Added by software-properties deb http://mirrors.aliyun.com/ubuntu/ xenial universe deb http://mirrors.aliyun.com/ubuntu/ xenial-updates universe deb http://mirrors.aliyun.com/ubuntu/ xenial multiverse deb http://mirrors.aliyun.com/ubuntu/ xenial-updates multiverse deb http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse deb-src http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse #Added by software-properties deb http://archive.canonical.com/ubuntu xenial partner deb-src http://archive.canonical.com/ubuntu xenial partner deb http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted deb-src http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted multiverse universe #Added by software-properties deb http://mirrors.aliyun.com/ubuntu/ xenial-security universe deb http://mirrors.aliyun.com/ubuntu/ xenial-security multiverse
	 5.具体步骤：
	 5.1 cd /etc/apt/ 
	 5.2 ls 
	 5.3 sudo mv sources.list sources.list_bak
	 5.4 使用记事本（或者Vim等其他编辑器）修改源列表 sudo gedit sources.list
	 5.5粘贴所有阿里云源地址，保存，注意先将以前的全部删除，然后再粘贴。
	 6. 然后再升级更新列表 sudo apt-get install update  
	 7. sudo apt-get install upgrade
	 8. 接下来就可以畅通无阻的下载自己想要的了。