How can I download iOS 13 for iPhone 6s?

Are you sure that your iPhone is a model 6s and not a 6 ?

Should be the former (6s), these are the instructions to update it to the latest iOS version 14.2:--> Update your iPhone, iPad, or iPod touch – Apple Support

Should be the latter (6), it cannot support iOS versions beyond 12.4.9, because of hardware incompatibility.

Regards

Giulio

you can't it haven't been signed for ages, you will have to get latest