Updated

Hello,

There are different ways to ensure the highest iOS that your device will support is installed on your device.

This support article has instructions about updating an iPhone or iPad wirelessly --> Update your iPhone or iPad - Apple Support

You can also use Mac and PC computers to update system software:

Update iOS on iPhone - Apple Support

When updating, your device will update to the highest version currently available that it will support, or it will just say it is up to date and won't install anything. Apple indicates ( Unable to update my iPhone 6 Plus from 12… - Apple Community ) I need to point out the obvious and state that if you want to run a higher system version than your current device will run, that you need to buy a newer device.

- Any equipment currently running iOS 12.5.7 can't be updated to anything higher.

- If you have an iPhone 5S, 6 or 6plus, the highest you can currently only update to is 12.5.7

- If you have an iPhone 6S through 7, or SE(gen. 1), you can currently only update to 15.7.5

- If you have an iPhone 8 or higher, you can currently only update to the latest iOS, 16.4

Note that iOS 13 isn't available to anybody. If your phone will support it, you'll go to the next highest version.

It depends on your unidentified iPhone model.

See this compatibility chart:

The Facebook app will not run on iOS 12.5.7 or older.

Try using the web version of Facebook.