Update iOS

Your iPhone 6s should be able to update to iOS 15.7.8. The instructions are - select Settings > General > Software Update. If your iPhone is eligible for an update, you will be prompted to do so. If you have problems, please check this link:

Update your iPhone or iPad - Apple Support

Your tag line says that you have an iPhone 6s running iOS 13. The Related Article suggests that you were reading about iOS 16.6. Your phone cannot run iOS 16, but, as Carol B. says, you can update it past iOS 13.