You have multiple ways to do that.
First way is to poll the server to ask for update. This solution isn't really efficient and lot of params are had to choose: Time to update-> will affect battery and re-activity.
Second way is to use Google Cloud Messaging(GCM). It provides a solution to push data from server to user. It's really simple to implement. Check android documentation:
http://developer.android.com/guide/google/gcm/index.html[
^].This will optimizes battery and you could receive push even if your app is off.