How to update Silverlight? It doesn't do that automatically for some reason.
Once installed, Silverlight should obtain updates through the Microsoft Update. In case it doesn't do that, re-install the plug-in, but this time make sure to tick the Check for updates automatically at the end of the installation or simply download the plug-in from the official website and install it. It should update the plug-in as well.
Download page: https://www.microsoft.com/getsilverlight/get-started/install/default.aspx