Push Notification inside App while user started entering in to App

I’m looking at pushing notifications for my app users when they open app while landing on the home page. The solution what I’m looking at is … when ever there is a new update available in playstore or istore… I would like to send a notification popup to respective users who hasn’t downloaded yet to update app. In few cases the popup may be a mandatory to update or in few cases its a choice to users to update or proceed. Such push notification is what I’m looking at. Its bit urgent.