iPhone 6s plus update

Are you sure you have a 6s plus? If it is telling you that you are up to date when you try to update it probably is a 6 Plus. Go to Settings > General > About > Model and post the two numbers there (tap on the number to get the other one) and post them here.

If your phone is running iOS 12.4.4, you have an iPhone 6 Plus, not a 6S Plus. It cannot be upgraded further.