Latest software update iPhone will try to install this update later tonight 20 days complete but not installed this update

The iPhone cannot install that update, it is not the latest one. You need to see this support document to reset your phone and then download the latest update, iOS 13.2.2. Get help with over-the-air iOS updates - Apple Support

It looks like your phone has automatically downloaded a previous update, which you never installed. 13.1.3 isn't the latest anymore, which is why you cannot install this version any longer. The latest is 13.2.2.

You would now need to delete the downloaded update first, go to Settings --> General --> iPhone Storage.

Search for a file called "Update" or "iOS 13.1.3" and delete that file. It has the same icon as the Settings app.

Then download the latest iOS 13.2.2 instead and install it. (Settings --> General --> Software Update).

Another solution is to update with iTunes on a computer.