The steps
- Connect to Wi-Fi and plug the device in (or keep at least 50% battery).
- Open the update screen:
Settings → General → Software Update - Download the update, then install it. The device restarts once; your apps and data stay in place.
iPhones and iPads can install updates overnight automatically: enable Automatic Updates on the same screen.
No update available?
Two possibilities: your device is already up to date — or it has reached its end of support and will never receive updates again. Check your exact model: