Open the Settings app
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
Keep your iPhone connected to Wi-Fi and power during the update
Wait for the download to complete
Tap Install Now or let it install automatically
Enter your passcode again if prompted
Wait for your iPhone to restart and finish updating