How do you get iOS 13.3.1?

To update your iPhone wirelessly -

Go To Settings-->>General-->>Software Update and your iPhone will generate a software update request from Apple. If there is an update available for your iPhone then it will begin the process of downloading and installing it.

Here is more information on this Update your iPhone, iPad, or iPod touch - Apple Support

Good Luck.

Axel F.

It's not available yet.

You apply to the beta program. 13.3.1 is not released yet.

This won't get you iOS 13.3.1 if you are not a developer.