Why is my iPhone 6plus still iOS 12.5.6?
Why is my iPhone 6plus still iOS 12.5.6 and how can I get iOS 15?
iPhone 6 Plus, iOS 12
An iPhone 6/6plus can't be updated beyond 12.5.6, it doesn't have the hardware needed to run anything higher.
If you need a higher iOS version, you will have to purchase a newer phone.