Buyers of themes that contained Visual Composer will receive emailed instructions, if you haven't already, soon on updating to the secure version.
Edit: If you've bought a theme containing Visual Composer, you will now be able to download the secure updated version from the item page directly on CodeCanyon (once signed in). This will only be temporary, so please update your item soon.
Please also be aware that, unless you've directly purchased Visual Composer, your original theme purchase license from ThemeForest only permits use of this plugin with the purchased theme (ie. this is not providing a stand-alone license for Visual Composer).
If you need instructions for updating the plugin, you should be receiving the previously mentioned email soon, which includes those update instructions.