If the app seems frozen, all you need to do is press the home button two times, and once you see the list of apps, find this one, and swipe up to close it, and open it again. Hopefully, it will work just fine.
Go to the App Store, and see if there are available updates for the Facebook messenger.
Find the Facebook messenger icon on the Home screen. Tap and hold, until an X shows up. Click on it, and the app will be deleted. Go to the App Store, and download and install it again.
By pressing and holding the sleep/wake button, you will see the red slider. Just drag it, and turn your device off. Use the same buttons, and turn the phone back on.
From time to time, you should check if the new iOS version is out. If it is, download it and install it.