Iphone 6+ , when I go to settings, software update, the phone says iOS 12.5.5 your software is up to date. Automatic update is on. I want to update to iOS 15 or something better than 12.5.5.

lhoward108 wrote:

Can an Iphone 6+ be upgraded to iOS 15 or something more up to date than 12.5.5?

No.

The hardware is insufficient for anything past iOS 12, and no hardware upgrades are available.

Your choice is a newer (used, refurb) or new iPhone.

Any recent iPhone will be substantially more capable than an iPhone 6 Plus, as well.