iPhone 6 software update

You don't because you can't. iPhone 6 was released 8 years ago, is no longer supported and lacks the hardware required to be updated past iOS 12.5.6. Time for a newer, more modern iPhone at this point.

How do I update an older iPhone to a newer IOS when the phone indicates it's up to date?

Unfortunately, you don't.

iOS 12.5.6 is as far as the iPhone 6 can go, due to hardware limitations on the phone.

If you need to run a higher iOS, you will need a newer phone.....an iPhone 8 or higher.