how to upgrade iphone 6 to ios 14

You can't. The highest iOS version which a nearly 7 year old, out of date iPhone such as iPhone 6 is iOS 12.5.2.

Since you clearly need the newer iOS versions, it's time to retire that old, no longer supported phone in favor of a newer more modern iPhone which can run the latest iOS versions.

You can not. The highest possible for that model is 12.5.2.

You can't. It's not supported on a 6.

Same answer.