The tech giant has confirmed any users who haven't downloaded iOS 10.3.4. by November 3 would be locked out of features including email, web browsing and iCloud.
In an official support post, the company said: "It's always a good idea to keep the operating system on your iPhone updated.
"If you have an iPhone 5, it's especially important to update your device's software wirelessly or using your computer before November 3 to maintain accurate GPS location and to continue to use functions that rely on correct date and time including App Store, iCloud, email, and web browsing.
"If the update to iPhone 5 is not completed by November 3, 2019, you will be required to back up and restore using a Mac or PC in order to update because over-the-air software updates and iCloud Backup will not work."
The firm also confirmed the iPhone 4S and other older versions of the iPad also need the latest software to ensure GPS services will continue to work.
The model was launched back in September 2012, was the first iPhone to feature Apple's lightning charger.
The company says around 70 million handsets were sold, with two million preorders over the first 24 hours of its release.