Apple has started sending notifications to legacy iPhone 5 owners to install iOS 10.3.4, in order for their devices to keep functioning properly when it comes to date and time:
Starting just before 12:00 a.m. UTC on November 3, 2019,* iPhone 5 will require an iOS update 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. This is due to the GPS time rollover issue that began affecting GPS-enabled products from other manufacturers on April 6, 2019. Affected Apple devices are not impacted until just before 12:00 a.m. UTC on November 3, 2019.
*You should update your iPhone 5 before then based on your local time zone. You can use the map on timeanddate.com to determine the difference between UTC and your time zone.
Back in July, Apple issued a similar iOS update for 2012 and older devices for the same issue, including the iPhone 5.
Now, if users do not update by November 3, 2019, their over-the-air software updates and iCloud Backup will not work, and they’ll be forced to rely on old Mac or PC backups or restores, if available.
To update your iPhone 5, go to Settings > General > Software Update.