To give a comprehensive answer. There are basically two ways you can go here. Depends on where you want to put the responsibility.
App checks for new versions on server
Using this approach, you would add a method to your app that is being called on every launch to compare the current version of the app (wherever you want to save that) with the version stored on the server. To achieve this, the server must implement a small web service that returns the latest version as a string or something else very simple. No rocket science there.
Server pushes new version information to app
This way you implement more code on the server-side to do a Push Notification to your app which informs the app about the new version. Advantage here is: Less client-side code and less effort to check, because the version check does not happen every time the app is started but instead only once the new version is actually released.
