How to cancel Windows Update?

Open Command prompt with Elevated privileges

taskkill /im wuauclt.exe /f

This will use taskkill to "Ends one or more tasks or processes" searching for the image name wuauclt.exe which is the windows update process

To check if it's running and for more info about the process you can run

tasklist /v | findstr wuauclt.exe

This will output the authority that called the process so you can understand why you can't touch it with you current privilege level

Here is an alternative way, without the command line.

First, when accessing the update options through Metro by doing mouse to upper right corner->Settings->Change computer settings->Update/Restore, there is not even an "abort" button to press.

The Windows Update options have to be accessed through the classic desktop Control Panel, where you promptly discover that even though you are a user with Administrator privileges, you can't press the abort button. My guess is that this is a bug where MS forgot to add whatever is needed for that small shield to be added to the button and check user privileges.

Instead, go to Control Panel->Services and scroll down to Windows Update service. Right-click and press Restart, and hopefully you can now select/deselect which updates to install.