You're not the only one. See if https://www.google.com/?#q=KB3172985+fails+to+install helps you to solve it.
I've seen a few methods in links on the first page that might work. But it doesn't seem easy.
The past few times my computer has required a Windows update, it fails to complete the update and tells me it is undoing changes upon startup. I am running Windows 10 on my computer and the update is KB3172985. The update keeps trying to install itself almost every time I shut down/start up my computer and fails time after time. If I didn't provide enough info, let me know what you need to know about my computer in order to solve my problem. Thank you in advance.