I won’t update my iOS

If you are telling us you will not update your iOS, then that’s ok. You do not have to.

But I suspect you are asking why you cannot update your iPhone 6 Plus to an iOS version higher than 12.5.7. The reason is that this version is the highest your iPhone can support. To get a higher iOS version, you will need to buy a new iPhone.

Unfortunately, that is not possible. That model can not be updated further.

iOS 12.5.7 is likely the final update for the iPhone 6 Plus. No more updates.