Hw to update software

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. 

iPhone 6s is compatible with iOS versions up to 15.x. That excludes iOS 16.x.

Here is the description on how to update: Update your iPhone or iPad - Apple Support