Update my iPhone

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. 

Your tag line says that you have an iPhone 6 Plus running iOS 12. The iPhone 6 Plus cannot run iOS 13 or later. If you are running iOS 12.5.7, you're already running the most recent update available.

The text of your post says you want to upgrade an iPhone 6s. The 6s and 6s Plus are later models than the 6 and 6 Plus. They can run iOS 15.7.8, but not any version of iOS 16.