See, GPS technology needs to keep an eye on the date and time to tell your location -- it tracks the date by counting the number of weeks and storing the values as 10-bit figures. Around every 20 years, that 10-bit week number goes back to zero, causing issues for older devices not designed to handle the change. The most recent rollover happened on April 6th, which means Apple devices that can't install the latest iOS versions could be affected by the issue.
Apple's patched mobile OS will need to be installed before November 3rd. The company explained to this ensures GPS and clock work properly after that date, and failing to do so will impact performance due to a manufacturing fault. For the iPhone 5 and fourth-generation iPad, the update will show up under About in Settings as iOS 10.3.4. Meanwhile, the update will show up as iOS 9.3.6 for the iPhone 4s, first-gen iPad mini, iPad 2 and third-gen iPad. Check out Apple's newly posted instructions right here.
Update 7/23/19 11:26AM ET: This post has been updated to clarify that Apple says the patch will need to be installed by November 3rd, not that the patch will be available on that day.