Reinstall VirtualBox 6.0 on Ubuntu 18.04

1. Remove the old version of virtual box. (Note the * is required, because it will remove all the virtualbox related packages)

sudo apt purge virtualbox*

2. Execute the following commands to install.

sudo apt update
sudo apt upgrade
wget -q https://www.virtualbox.org/download/oracle_vbox_2016.asc -O- | sudo apt-key add -
wget -q https://www.virtualbox.org/download/oracle_vbox.asc -O- | sudo apt-key add -
sudo add-apt-repository "deb http://download.virtualbox.org/virtualbox/debian bionic contrib"
sudo apt update