If the download does not complete:
- If it does leave what looks like a "complete" installer file in the /Applications folder, delete it, and try downloading it again.
- If it does not leave an installer in the /Applications folder, it may leave an incomplete/corrupted installer in the /Library/Updates folder. To help locate the partial installer package, try using the following command in the Terminal app:
- lsof | grep <the installer filename>
If neither of the above is the case, try booting up your Mac in Safe Mode, and then, try updating your Mac again.
Ref: Use safe mode to isolate issues with your Mac - Apple Support