Connect your iPhone to Wi-Fi
Plug your iPhone into power
Open Settings
Tap General
Tap Software Update
Wait for your iPhone to check for updates
Tap Download and Install if an update is available
Enter your passcode if prompted
Agree to the terms and conditions if prompted
Wait for the update to download
Tap Install Now if needed
Keep your iPhone connected to power during installation
Wait for your iPhone to restart
Check Settings > General > Software Update to confirm it is up to date