I want to update my iPhone 6 to iOS 14

Unfortunately, the iPhone 6 is not compatible with iOS 13 or higher.

The highest possible for that model is iOS 12.5.6.

Sorry, not possible. The iPhone 6 lacks the hardware, RAM and CPU to updated past iOs 12.5.6.

Also, iOS 14 is no longer available for any device. For supported models the only options available are iOS 15 or iOS 16.