A
Welcome to Android Central! Are you asking how to update the Facebook app? Did your phone come with Google Play Store preinstalled? If so, open it, look for Facebook, and tap Update (if an update is available).
Welcome to Android Central! I'm guessing that you're not the original poster on this thread, so which phone do you have? Try going to Settings>Apps, select Facebook, then Storage, then Clear Cache/Clear Data. Then open the app again and log back in. Does the problem persist?