Why won’t software update on my phone?

Take a look at the article below:

Get help with over-the-air iOS updates - Apple Support

If you still can't install the latest version of iOS, try downloading (fontvilla) the update again: Go to Settings > General > [Device name] Storage. ... Tap the iOS update, then tap Delete Update. Go to Settings > General > Software Update and download the latest iOS update.

If you're looking for the iOS 13 update, it won't be out until later today. Be patient.