Generally the store through which you distribute your app provides that update automatically to your user, so your app does not need to do anything. If you want to handle that yourself, you will have to find a third-party service or setup your own server where you could store the pertinent information, and make sure to keep a marker in your application to compare what version you're at versus what version your server believes to be the most recent version.
It's possible someone has already written a Cordova plugins to do this sort of thing. You'll have to use your favorite search engine to search for something like "cordova phonegap plugin update notification" or something similar.
