chmod 777 /etc/apt/sources.list 更改文件权限使其可编辑；
    cp /etc/apt/sources.list /etc/apt/sources.list.bak
    vi /etc/apt/sources.list 进去之后把里面的东西删除，放入如下内容
    deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial main restricted universe multiverse
    deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates main restricted universe multiverse
    deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse
    deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security main restricted universe multiverse
    之后保存退出
    之后执行 apt-get update
    此时会遇到问题
    # apt-get update
    Reading package lists… Done
    E: Could not get lock /var/lib/apt/lists/lock - open (11: Resource temporarily unavailable)
    E: Unable to lock directory /var/lib/apt/lists/
    出现这个问题的原因可能是有另外一个程序正在运行，导致资源被锁不可用。而导致资源被锁的原因，可能是上次安装时没正常完成，而导致出现此状况。
    解决方法：输入以下命令
    rm -rf /var/cache/apt/archives/lock
    rm -rf /var/lib/dpkg/lock
    之后就解决了

    20220617更新源文件
    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-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