What should I do with MacOS 10.12.6 update that the App Store forgot it downloaded?
If you have a partially downloaded and/or installed update for Mac OS stuck on your Mac, you must re-download and reinstall the update on your Mac. You can do this in one of two ways. Either way will accomplish the same thing, with the same results, with no practical difference between either method.


Download the update for Mac OS via the mac app store. 
A second option is to download your update outside of the mac app store by using Apples official download site located at https://support.apple.com/kb/DL1970?locale=en_US . For people who may read this in the future and experience a possible broken link in the above reference, just go to the apple page (or google) and search for Mac OS Combo update download. From Apples website, search or follow the links to the proper (or desired) version (ex:10.13.6) of your update, and download it. The downloaded file should be in .dmg format. Then double click the file, and install the corresponding app from the installer window.


Special Note in regards to Official Apple Software updates:

DO NOT download apple software or updates from a non-apple source.

Once the freshly downloaded update is installed, Mac OS should clean up any leftover files including the "corrupted" or "Partially downloaded" update. 

If this is your only problem you are having with Mac OS, there is no reason to do a full reinstall, even if you are fully backed up. Lastly, I must stress to users how important updates are (primarily for security reasons). You should never get lazy and ALWAYS take the time to install new updates as soon as they arrive in the app store. This will keep you protected, and defended against hackers/rootkits/virus's/trojans/botnets/etc. etc. So please, don't wait, just update the Mac. If for some reason those partially downloaded update files are still there after the update, and if Mac OS doesn't clean up after itself, let me know.