Why can I update my phone

Hello ~ Take a look here and make sure your iPhone of your iPhone model:

Identify your iPhone model - Apple Support

If you have a 6 Plus your iPhone is running the highest iOS it is capable of running. If you require or would like to run latest iOS versions you can go to black line above and click on iPhone to see the newest devices.

~Katana-San~

The iPhone 6 Plus cannot be upgraded beyond iOS 12.x.x.x. It lacks components to run newer software.