How can I upgrade my iOS 12.5 to iOS 15 on a iPhone 6plus

Unfortunately, that model is not compatible with iOS 13 or higher.

The highest possible for that model is iOS 12.5.5.

How can I upgrade my iPhone 6plus with iOS 12.5 to iOS 15

You can't. The hardware on the iPhone 6 does not meet the requirements of the software in newer operating systems.

Or, stated simply.........the iPhone 6 is too old.