登录root用户

su

#先备份原来的源列表文件
mv /etc/apt/sources.list /etc/apt/sourses.list.backup

#新建源列表文件
vim /etc/apt/sources.list

#加入源地址（推荐用阿里云源）
deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse

#更新软件列表
sudo apt-get update

#更新软件（把已安装的软件与刚刷新的软件列表中的版本进行对比，如果发现已安装的软件版本太低，就会提示更新）
sudo apt-get upgrade