Why I can’t update my phone

If you have an iPhone SE 1.Gen, it can be updated to: iOS 15.7.3. Try closing the Settings app completely and restarting your iPhone. Make sure you are connected to WiFi and also connect your charger.

Go to Settings > General > Software Update and try again. Try not to use your iPhone in the meantime.

You can read more tips in this article:

If your iPhone or iPad won't update - Apple Support