First thing first, don't touch on "LATER" or "INSTALL NOW", just touch the home button whenever you see above popup.
And, good news, I've found a way to get rid of the downloaded (but not installed yet) dangerous update.
Download and install KingRoot 4.1. You may need to give it many tries to eventually get a successful root and this V4.1 gives you the highest chance. If it no longer want to try, clear its data, force stop it and start it again. During the rooting process, don't do anything and just wait. Removing unnecessary apps may also help the success rate, especially those conducting lots of background activities such as TogicVideo.
Now download and install a terminal emulator (I use JuiceSSH). Make a new connection to local and type in "su" once you are in the terminal. KingUser will ask you and you select "Allow"：
Now you have the root access, type "ls -l /cache" and you'll see there the downloaded update file, Blur_Version.22.41.28.surnia_verizon.verizon.en.US:
Type "rm /cache/Blur*" to remove the downloaded update. Type "ls -l /cache" again to make sure you no longer see it. Reboot, you'll happily see this:
It is too early to tell whether the phone will get the update again or not, but at least peace for now.
[update] The next morning, the system update notification showed up again. This time, I simply root again and removed the downloaded zip in the cache folder. From now on, I'll just touch the home button whenever the update pops up. Not a big deal, because the popup does not show up too frequently and I know no damage if I accidentally touch the wrong button.
[update] Permanently block system update and other notifications.