How can I download this update ?

Hi mayamu66!

The latest software available for the iPhone 6 is iOS 12.5.7, which the device is already running. Your iPhone 6 is on the newest software available for it.

Take Care!

iPhone 6, iOS 12

iOS 12.5.7 is the latest iOS security update for iPhone 6.

Sorry, iPhone 6 can’t be update to a higher iOS (13 and later).