Updating your iPhones, iPads and iPod touches to iOS 10.2.1 might be the most secure thing you do today. That’s because the new update includes a wide range of security fixes that protects against very bad attacks.
Over a dozen vulnerabilities are listed in the 10.2.1 update, including two where a malicious app can execute code within the kernel, which means it can take complete control of your iOS device. Seven vulnerabilities are listed for WebKit, the engine that powers mobile Safari, including two fixes that list how rogue websites can execute code on the impacted iOS device.
If you own an iOS device, grab the update to iOS 10.2.1 by going to the Settings app, then General, Software Update. You must be plugged into a power source for your device to update, and as always, we recommend you back up your device before updating.
Apple has also released updates for watchOS, macOS and tvOS, which are focused on bug fixes and performance improvements.
Is IOS 10.2.1 still applicable/suitable to iPhone 5?
because my iPhone 5 is already lagging in IOS 10.1.1.