This post gives step-by-step instructions for upgrading VirtualBox from 5.1.14 to 5.1.30.
You may want t completely restart your Windows machine before attempting this.
1. Click the link: http://download.virtualbox.org/virtualbox/5.1.30/VirtualBox-5.1.30-118389-Win.exe.
2. Save the file to C:\Users\your_username\Documents\Installers
3. Double-click it.
4. Click Run
5. Click Run again
6. Click Next
7. Click on the VirtualBox Application box and select Entire feature will be installed on local hard drive the click Next
8. Click Next again
9. The next step warns that the installation may reset the network device. Don't worry, the network reset will occur during install after you click Yes (and Install in the next step)
10. Before clicking Install:
10.a If you didn't close VirtualBox you'll see a window that says that its still open. Close VirtualBox and click Retry.
10.b There may additional files still in use. The installer may list a few numbers. These are PIDs.
10.c To figure out what to close you can type Control-Alt-Delete. Click the Processes tab. Click View then click Select Columns...
10.d Select the PID (Process Identifier) checkbox
Back in Windows Task Manager, make sure the Show processes from all users checkbox is checked. find the numbers listed (1868, 5604, 7084 in the example above). You can click on the PID column to sort the numbers. For each number click End Process
If any PID is a SYSTEM PID you may get an Access is denied error
If you get this, stop the installation, reboot the computer and try again.
11. Click Finish and open Oracle VM VirtualBox
Difference between 5.1.14 and 5.1.30 detailed at: https://www.virtualbox.org/wiki/Changelog-5.1.