On my Ubuntu 16.04 machine, my Virtualbox 5.0x told me there was an update. I clicked the link and downloaded the update (Virtualbox version 5.1.24). When I clicked on the downloaded .DEB file it launched the UBUNTU SOFTWARE CENTER and I clicked on the INSTALL Button. Then... Nothing... It flashed to INSTALLING, then stopped and offered up the INSTALL Button again.
I few Google searches later and I found the following Terminal commands that worked. I was a little scared of these since the first command uninstalls your 5.0 version of Virtualbox because I have several paid for apps running on my Windows 10 virtual but this worked perfectly.
Now let's open a TERMINAL windows and get this update done!
Copy and paste the following five (5) commands into your TERMINAL window (one at a time). Provide your password when asked:
sudo apt remove virtualbox virtualbox-5.0 virtualbox-4.*
sudo sh -c 'echo "deb http://download.virtualbox.org/virtualbox/debian xenial contrib" >> /etc/apt/sources.list.d/virtualbox.list'
wget -q https://www.virtualbox.org/download/oracle_vbox_2016.asc -O- | sudo apt-key add -
sudo apt update
sudo apt install virtualbox-5.1
Once your done, when you first run your virtual, you'll be asked to update your GUEST ADDITIONS which also worked just fine.
NOTE: This adds the official Virtualbox Repository to your software sources so you should get auto-updates after following these steps.