How to delete partially downloaded updates in High Sierra? I'm trying to update my Macbook Pro to the latest security update for High Sierra but the download doesn't seem to finish and is working in a really weird way. I tried to cancel the current update, and it does cancel, but when I try to start it again, it doesn't start from the very beginning and shows an entirely different progress than what it showed before I cancelled it. For instance, I cancelled the download at around 700mb, but when I tried up update again, it shows that only 300mb has been downloaded.
I want to start the download from the start, from 0%, which I think will fix the issue but I can't seem to do that, too. Can anybody please help me? I tried to wait it out already, but it just gave me a connection error around 1.6GB. When I tried to start again, it started at around 738mb.
Sorry, but I only have a partial answer to your question:

First off I would try a different approach and download from here: https://support.apple.com/downloads 

Also a safe boot (pressing and holding the shift button at boot until the Apple symbol appears) with an installation attempt there often succeeds.

As far as I know High Sierra downloads tmp system updates to a file, or files similar to this: /private/var/folders/zz/zyxvpxvq6csfxvn_n00000s0000068/C/com.apple.SoftwareUpdate/CFNetworkDownload_XxxXxX.tmp 
...then assembles them into a subdirectory, within...
/private/var/folders/zz/zyxvpxvq6csfxvn_n00000s0000068/C/com.apple.SoftwareUpdate/swcdn.apple.com/content/downloads/...before moving them to... /Library/Updates/. 

You can always boot into "Recovery", which should let you browse all directories in Terminal (to be found under the "Utilities" menu).