How to delete partially downloaded updates in High Sierra?
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).