How do I update my IPhone 6 to the new IOS 15?

You cannot. The iPhone 6 cannot run iOS 13 or higher as it is too old.

The iPhone 6 is not compatible with iOS 13 or higher.

The highest possible for that model is iOS 12.5.5.

You don’t. Your 7+ year old iPhone’s hardware is not capable of running an iOS version higher than 12.x.