How to install this update on 6s ??

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

Did you click on the blue links he provided and follow the instructions?

What version are you currently running -> Settings > General > About > Version