How can I download software ios13.1

You cannot. Any device capable of running iOS 13 is also compatible with iOS 14. So any such device will only be able to update to the most recent release which is iOS 14.4.2.

If your device is running iOS 12 or older and says it is up to date, that means there is no newer version of iOS compatible with your hardware.

That iOS version hasn't been available in years.

Unless you have a 6s, or higher, you can't update your version.