Dark mode in iphone6

Never. The iPhone 6 is too old and incapable of running iOS 13. iPhone 6 doesn't have enough RAM for iOS 13 and your 5 year old iPhone is now unsupported, it can only recieve more iOS 12 security updates.

If you want iOS 13 / the Dark Mode, you need to buy a new iPhone. iOS 13 is supported by the iPhone 6s / SE and newer.

No way to do so since iPhone 6 can't run iOS 13. Dark Mode os a feature of iOS 13.