There are two ways to update your iPhone software: directly on your device or using iTunes on your computer.
To update on your device, go to Settings > General > Software Update and tap Download and Install if an update is available. You can also turn on Automatic Updates to let your device download and install updates overnight while charging and connected to Wi-Fi1.
To update using iTunes, plug your iPhone into your computer using a Lightning cable, open iTunes, click the iPhone icon, click Check For Update and then click Update if an update is available.