How can I update my iPhone 6s

If your iPhone 6s is not up to 15, its highest iOS as mdp1101 mentioned, then you can update it by selecting Settings > General > Software Update. If your phone is eligible for an update, you will be prompted to do so. You can find more information in the link below. But if your phone is not eligible fro an update - it is an older iPhone - then there is nothing you can do to get a higher iOS other than buying a new iPhone.

Update your iPhone or iPad - Apple Support

Hello.

Unfortunately, the iPhone 6s no longer supports the newer versions of software and instead can only run software up to iOS 15.

Regards,

mdp1101