How to update iPhone 6s to the latest iOS version?

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. 

Do you have an iPhone 6, or iPhone 6s?

The iPhone 6 cannot be upgraded past iOS 12.5.6. On the other hand, the iPhone 6s can be upgraded to iOS 15.7.x.