How to update my iPhone 6s to the latest 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. 

Go to settings - general - software update and start there. An iPhone 6s can be updated to iOS 15 only.

Software update and start