I want to update my iPhone

Hello,

There are different ways to ensure the highest iOS that your device will support is installed on your device. For an iPhone 6S or 6S Plus that's iOS 15.7.7

This support article has instructions about updating an iPhone or iPad wirelessly --> Update your iPhone or iPad - Apple Support

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 will run, that you need to buy a newer device.