How do I upgrade my iPhone 6s to the latest iOS version?

Sorry but your iPhone 6s's iOS cannot be upgraded past iOS 15. Currently it's 15.8.5, and any further updates will be decimal security and bug patch updates. Any later iOS will require a newer iPhone.

The iPhone 6s originally ran iOS 9 when it came out over a bit over 10 years ago. It received six major iOS updates, and you can update it to the latest release of iOS 15.

You cannot update it to – and will never be able to update it to – iOS 16, 17, 18, 26, or beyond.

iPhones 6S through 7, and SE(gen. 1), will only update to iOS 15.8.5

There are different ways to ensure the highest iOS that your device will support is installed on your device. When updating, your device will update to the highest version currently available that it will support, or it will just say it is up to date (and can't go any higher) and won't install anything. If you want to run a higher system version than your current device is capable of running, you need to buy a newer device.

This will work, assuming you’re not on the latest software (iOS 15.8.5)