Why are Apple releasing an unscheduled upgrade?
Well, according to Apple’s release notes that accompany all updates, iOS 9.3.4 is all about one (slightly vague) thing: “iOS 9.3.4 provides an important security update for your iPhone or iPad and is recommended for all users.”
It appears that some recent jailbreaking activity targeting iOS vulnerabilities has highlighted a flaw that may allow rogue apps to gain control of part of operating system. This upgrade is designed to fix this problem.
Which devices need to be updated?
iOS 9.3.4 is designed for the iPhone 4S or later, iPad 2 or later, iPad mini or later, iPad Pro range and the 5th generation iPod touch or later. Users will be automatically prompted to upgrade, but if that notification has not yet appeared for you it can be triggered manually by going to:
Settings > General > Software Update
Should I run the upgrade?
Apple recommend upgrading for ALL users, so unless you're planning to jailbreak your device, go ahead and run the upgrade!
With the iPhone 7 coming out in September, this may well be the last iOS update owners of the iPhone 4S, iPad 2, iPad 3 and iPod 5th generation will receive.