Want to download this iOS on my iPhone 6s

If your iPhone is a 6 Plus model (as it appears on your products list, below your question), it cannot be updated past ios 12.5.7 version.

If, on the contrary, it's a model 6s or 6s+, it supports iOS 15.8 version, no further.

As far as the instructions to update it are concerned, all the steps are listed in this support article:--> Update your iPhone, iPad, or iPod touch or, if you have problems, in this other article:--> If your iPhone, iPad or iPod touch won’t update

Regards

Giulio

Sorry no, not an iPhone 6 or 6 Plus. See why in this compatibility list:

My pleasure :)