How can I delete the downloaded software from my phone to download a iOS software of 13?

You can only upgrade to the highest available iOS version that your 6s supports which is iOS 14.6. Click on a link below for full instructions about how to update with wi-fi or 5G, or using a computer if you have storage or connectivity issues.

"Update your iPhone, iPad, or iPod touch" click here --> https://support.apple.com/HT204204

Or this video:

"How to update your iPhone or iPad" click here --> https://youtu.be/csJAEyvvruY

Go to Settings/General/About and read what it says for Model Name. My guess is that it’s an iPhone 6, not a 6S. If that’s the case it cannot be updated higher than the most recent release of iOS 12.