To install the update on your iOS device, launch the Settings app, then tap General > Software Update. With previous versions of iOS, there are vulnerabilities that can allow someone to hijack the iPhone or iPad and control or monitor it remotely.
If you device isn’t upgraded, there are numerous apps that a hacker could monitor or use, including iMessage, Gmail, Viber, Facebook, WhatsApp, Telegram, and Skype.
This critical update is available from Apple. To install the update on your iOS device, launch the Settings app, then tap General > Software Update. You also can update within iTunes with your device connected to your Mac computer.
Read more from Macworld:
http://www.macworld.com/article/3112318/ios/apple-advises-immediate-update-to-ios-9-3-5-after-discovery-of-targeted-iphone-spyware.html