在百度搜索–>ubuntu镜像站–>选择清华的镜像源–>进去选择ubuntu—>选择ubuntu的版本（这里以16.04的为例）–>复制里面的内容：（内容如下，不过一直在更新，我们可以安装上面的步骤去搜索即可）
1.先备份系统的/etc/apt/sources.list
cp /etc/apt/sources.list /etc/apt/sources.list.back
2.打开文件，替换里面的内容,就 OK了
gedit /etc/apt/sources.list
例如阿里源：
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-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse
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-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse