To manually start the update, unlock your iOS device and go into Settings > General > Software update.
There, you will see the iOS 11.3 software update, a big update weighing 2.18GB. Make sure you are connected to a Wi-Fi network and grab a cup of coffee or do something around the house as it will take a while for the update ...
from PhoneArena - News https://www.phonearena.com/news/Apple-releases-iOS-11.3-software-update-battery-health-tool-new-animoji-and-more_id103697
via Telephone Engineers