How can I update my iPhone 6s to the latest iOS version?

Apple no longer provides new iOS software releases for the iPhone 6s. The maximum iOS version you can install will be 15.8.3 which you should be able to achieve via Settings > General > Software Update.

rafique81 wrote:

Halp me my iPhone 6s new iOS update 16

That is not possible. The 6s can not run anything later than iOS 15.

The iPhone 6s cannot update to iOS 16.

You need to have at least an iPhone 8 or iPhone X to run iOS 16.

So, you need a new phone if you want to run iOS 16.