If you use Android 4.1 and higher:
Settings > Device > Application Manager > I Am Hero> Show Notifications ON/OFF
If you use Android 5.0 and higher:
Settings > Apps > I Am Hero > Show Notifications ON/OFF
If you declined to receive notifications in the first place, try to reinstall the game and agree to get notifications in the tutorial. Do not be afraid; you will not lose your progress if you follow these steps:
1. Write down your ID somewhere so our Support team can restore account manually if something goes wrong.
2. Make sure you are connected to your Facebook account before deleting the game.
3. Delete and reinstall I Am Hero from Google Play Store.
4. Launch it and agree to notifications in the tutorial.
5. Navigate to Options and log in via Facebook