Use Nick Lockwood's iVersion library. It's whole purpose is to notify the user when new updates are available.
iVersion is a library for dynamically checking for updates to Mac/iPhone App Store apps from within the application and notifying users about the new release. It can also notify users about new features in the app the first time they launch after an upgrade.
Purpose:
The Mac and iOS App Store update mechanism is somewhat cumbersome and disconnected from the apps themselves. Users often fail to notice when new versions of an app are released, and if they do notice, the App Store's "download all" option means that users often won't see the release notes for the new versions of each of their apps.
Whilst it is not permitted to update an App Store app from within the app itself, there is no reason why an app should not inform the user that the new release is ready, and direct them to the App Store to download the update.
And if your app is not on the App Store, either because it's an in-house/enterprise iOS app, or a Mac app delivered to customers outside of the store, you can't use the App Store update mechanism anyway.
iVersion is a simple, zero-config class to allow iPhone and Mac App Store apps to automatically check for updates and inform the user about new features.
iVersion automatically detects when the new version of an app is released on the App Store and informs the user with a helpful alert that links them directly to the app download page.
Or if your app is not on the store, iVersion lets you specify a remote plist file to check for new releases, and a download URL where users can get the latest release.
iVersion has an additional function, which is to tell users about important new features when they first run an app after downloading a new version.
These excerpts were taken from the Github page here, where you can download it. There is also a comprehensive tutorial on installing and configuring iVersion, so I recommend that you check it out.
Hope this helps!
