Check whether your iPhone supports the iOS version you want to install
Back up your iPhone to iCloud or a computer
Connect your iPhone to Wi-Fi
Plug your iPhone into power
Open Settings
Tap General
Tap Software Update
Tap Download and Install if the update appears
Enter your passcode if prompted
Wait for the download and installation to finish
Restart your iPhone if it does not restart automatically
If the update does not appear, check again later or use a computer with Finder or iTunes to update