Upgrade to iOS 14

No. But it can be updated to iOS 15.7, which is higher than iOS 14, so anything requiring iOS 14 will run just fine on 15.7.

Are you certain you have an iPhone 6s? Go to Settings > General > About > Model and Version

If you instead own iPhone 6, then no, it cannot be updated past iOS 12.5.6.

No. An iPhone 6s supports iOS 15.x, and you can only upgrade to the newest iOS the device can support. It will not support iOS 16.