iPhone 6s Plus update

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. 

An iPhone 6s Plus can run iOS 15.*. It can't run iOS 16 or higher.

If the phone is actually an iPhone 6 Plus (no "s"), iOS 12.* is the end of the line.