Is it still just checking, or is it downloading something?
If it's still just checking, you should be able to hit the Back button to leave it. If it's not visible, swipe up from the bottom. If it still doesn't work, restart the phone (Hold the power button in until it restarts - maybe up to 30 seconds, depending on the phone).
If it's downloading something, or has already downloaded an update, just let it finish, and make sure you plug the charger in if it goes below 40% and unplug it if it goes over 80%.