How can I download iso 13.0 for my iPhone 6s,.?

go to Settings > General > About and let us know what it says for iPhone Model and Software Version

If your iPhone is really a 6 and not a 6s, you will not be able to update beyond iOS 12.

If your iPhone is really a 6s, then you still cannot update to iOS 13, you would have to update to iOS 14

Update your iPhone, iPad, or iPod touch - Apple Support

Updating iOS: common issues and solutions - Apple Community

For an iPhone 6S you can update to iOS 14.3.

For an iPhone 6 you can update to iOS 12.5.

There is no option for iOS 13.