receive the Message about updating, go ahead and do it.
Go to the App Store, it's on the left hand end of the dock and looks like circle with the letter A in it. When it is launched, click on the Updates tab.
You may find that there are Security Updates that you need to install.
Install everything that the Software Update application finds
my imac 21.5 in late 2013 model already had El Caption and for some reason it keeps saying that i need to install el caption or either update it ( this is my first mac so kinda new to all this) but when i go to about this mac it say vel caption 10.11.4 is already installed and that what it try to download, any help of what to do?