How can I update my iPhone 6s Plus?

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. 

Latest iOS for a 6s Plus is Latest release of iOS 15.

6s Plus cannot be updated to iOS 16, if that is what you are really looking for.