Apple is set to release iOS 10 today. Before any major update or upgrade, you should make a backup of your iOS device. You can read about that on Apple’s How To Back up your iOS device page.
Without repeating all of the various new features introduced in iOS 10 here, simply visit Apple.
You an also watch short the film that bursts through some of the features.
To update your device, tap on Settings > General > Software Update. Run the update. You should access the update form a Wi-Fi connection and it would be a good idea to have your device plugged into a power supply.