You may have been in a situation where Visual Studio 2012 notification about available updates has popped up, but you’ve been too busy to do the update at that particular time. If you closed Visual Studio, the update notification is gone, and on starting up VS again sometimes it does not appear.
You can manually initiate an update, but it is somewhat hidden. To do a manual update, open Visual Studio and go to Tools -> Extensions and Updates. On this screen you’ll see any available updates that haven’t been installed. To install an update, select it and click on “Update”.