The technology giant said users who did not download iOS 10.3.4 by 3 November would be locked out of features that rely on the correct time and date.
This includes the App Store, email, web browsing and storage service iCloud.
While it is not the latest version of the operating system, it is the most up-to-date available for the model.
Users of older iPhones have also been advised to update their software in order to maintain accurate GPS location services.
iPhone 5 users have received pop-up messages in recent days urging them to update to the latest iOS.