Download the VirtualBox image from https://www.virtualbox.org/wiki/Downloads
Download the 16.04 Ubuntu image for VirtualBox from www.osboxes.org (the credential details for logging in are here: https://www.osboxes.org/faqs/)
Open VirtualBox, click the New icon, Select ‘Linux’ for type, ‘Ubuntu 64-bit’ for version (assuming you downloaded the VirtualBox 64bit vdi), click next a couple of times until you reach the ‘Hard Disk’ section and then click the ‘use an existing virtual hard disk file’ radio button
and browse for the vdi file you downloaded.
Log in using the credentials given (osboxes.org as password). After logging in it is recommended to install the guess additions (to be able to have things like shared folders and shared clipboard). Do this by selecting Devices->Insert Guest additions CD image… from the VirtualBox menu. This will add a CD icon to the unity bar on the left, meaning the guest additions CD has been mounted. Open a terminal window and type the following at the command prompt:
This will install the guest additions on the Ubuntu guest system. Ignore the #modprobe vboxsf failed# error at the end and restart the system via typing
sudo restart at the command line
After successfully installing the guest additions you can eject the CD (right-click the CD icon and select eject)
After logging in again, open the terminal window again and do:
sudo apt-get update
sudo apt-get upgrade
The above update and upgrade will take a few minutes so after it is done, it is recommended to export your image should you want to have run the updated VirtualBox image on another machine without going through the entire process above yet again. Do this by closing the window and selecting ‘Power off the machine’. Next, Open the Oracle VirtualBox manager. Select File->Export Appliance from the menu, select the name of machine and click Next a couple of times. The resulting file/files can then be imported to a different VirtualBox app.