The Windows Update KB3172985 is a Cumulative update for Windows 10 Version 1511 causes problems for many users when it fails to get installed after several attempts. Most of the time when a user lets it get installed, it gets stuck on its way to being 100 percent installed (between 50 to 60 percent) and the user is forced to restart. Only after you reboot multiple times, do you get the chance of booting into your operating system. This issue mainly occurs due to the corruption of update components.
Following the two methods below which should hopefully resolve this issue for you.
Method 1: Reset Windows Update Components
Press Windows key + X to invoke the menu above the start button.
Select File Explorer from the list.
In the file manager window, right click on the C:\ drive and select “Properties”.
On the General tab, click on Disk cleanup.
After a processing pop-up, the disk cleanup window should appear.
Under the Files to delete section, make sure Downloaded program files is checked.
Now click on the “Clean up system files” button present towards the bottom of the window.
Once done, go to the Windows folder on your C:\ drive.
Now go to the folder named “SoftwareDistribution”.
Then double click on the folder “Download”.
Delete all the contents of this folder.
Now press Windows key + A to invoke the action center.
Click on Settings from the list.
Now click on Update and security.
Scroll down and click on “Advanced options”.
Now click on the “Choose how updates are delivered”
Disable the option by setting it to “Off” if it’s currently on “On”.
Restart and try installing the update again.
Method 2: Install the Update Manually
If the Method above fails, try installing this update manually by downloading the stand alone package from here
If this method doesn’t help you in installing the update, then try the steps listed here