Hi everybody.
Sorry to answer kinda late, but after months of unsuccessfully installing the update I stumbled upon a good solution to this persistent problem.
If you follow the instructions properly you should have no issues in downloading the update and installing it.
Here it is:
Triple-click anywhere in the line below on this page to select it:
/var/folders/zz
Right-click or control-click the highlighted line and select
Services ▹ Reveal in Finder (or just Reveal)
from the contextual menu.* A folder should open with a subfolder named "zz" selected. Move that subfolder to the Trash. You may be prompted for your administrator login password. Restart the computer and empty the Trash.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select
Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You won't see what you pasted because a line break is included. Press return.
After you restart your Mac you can install the update as usual and the notification will naturally disappear.
I found this in this website: http://www.codeexplain.com/code/24931c7b-41a2-4b66-9f63-2e9974cb353b
I hope every Mac user gets this annoying problem solved as I did.
Good luck!