When your Mac connects to Apple to update software, there are several steps it takes to complete the process. The software update system uses—at minimum—four different servers to check, verify and download the updates available to your machine.
The first server in the process and server that your machine connects to is swscan.apple.com. (These servers are not user-clickable links.) This runs a check to see what is on your machine currently. In the second step, it connects to swquery.apple.com to check to see what is available for the unit. The final two steps include two other servers, swdownload.apple.com and swcdn.apple.com, that verify the location of the downloads and the content that is downloaded. The actual downloads are served by Akami content distribution servers.
During the process of the download, your system may stall and not complete the download process. There are a few local files—caches of the download—on your machine that may be causing the issue. If the download stalls in the download process the content is saved in a folder, /var/folders/zz. Removing the contents of these folders may resolve the issues related to the hang in downloading.
To remove the downloads that have started, you need to open Terminal and navigate to the folder mentioned above, /var/folders/zz. After navigating to the folder in Terminal using the cd command, you can look at the content by typing the command ls. You will see several files in the folder all titled zzz something. To delete these files—which is safe to do—type sudo rm -rf * in Terminal. This directs the machine to, as a super user, remove the content of the directory /var/folders/zz. You’ll have to enter your password for the command to remove the files.
After removing the files you may now try to run Software Update again. Now that these caches have been removed, the system will start a new download of the updates and this may resolve the hang you have been experiencing.