The update is exactly what it says it is, an updated iOS Push Certificate. The potential change is that iOS push updates may stop working if you don't install the update.
This has nothing to do with the iOS app. 3CX doesn't talk to the iOS app on the phones for push. 3CX notifies Apple, Apple handles the push to the iOS devices
Did you install from the 3CX ISO? Then you just click the update button after making sure you have a backup. The whole point of the 3CX ISO is they test/ensure compatibility.