how to download facebook but my ios is not verified

The Facebook app requires you to use iOS 13.4 or later.

If your iPhone supports iOS 15, or the latest version iOS 16, you must update your iPhone to use the Facebook app. If your iPhone does not support one of those versions, then you cannot download the Facebook app on your current iPhone.

Your iPhone 5S/6 can be updated to: iOS 12.5.7

Your iPhone 6S/7/SE 1.Gen can be updated to: iOS 15.7.3

Your iPhone 8 or newer can be updated to: iOS 16.3.1

You can check your current version here:

Settings → General → About

You can download and install software updates here:

Settings → General → Software Update

You can find more useful information here:

Update your iPhone or iPad - Apple Support