First check and see which software version you currently have: Settings > General > About> Software Version. If it is anything less that 13.4.1, follow the instructions below to update.
1. Make sure Wifi is on and connected (Settings > WiFi)
2. Plug the device into the charge
3. Go to Settings > General > Software Update > Downlad and Install.
4. Wait for the update to download. Once it is completed, it will ask to to install it.