Can I have iOS 15.6 or iOS 16 on my iPhone 6s?

Hello,

There are different ways to ensure the highest iOS that your device will support is installed on your device.

Any equipment currently running iOS 12.5.7 can't be updated to anything higher.

An iPhone 5S, 6, or 6plus, will only update to iOS 12.5.7

iPhones 6S through 7, and SE(gen. 1), will currently update to iOS 15.7.7

An iPhone 8 or later will only update to the latest iOS, 16.5.1

This support article has instructions about updating an iPhone or iPad wirelessly --> Update your iPhone or iPad - Apple Support Read the article for recommended criteria for being able to use wireless updating.

You can also use Mac and PC computers to update system software --> Update iOS on iPhone - Apple Support

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.

With your 6s you can have 15. Simply try to update and your device will install the highest it can install. 15.8 is a beta release, available to those in the beta program.