Does iPhone 6plus have dark mode if yes how

It's not available on your 6 plus, or any older phone as you can't run iOS 13 or later.

It does not.

The dark mode option requires iOS 13. The iPhone 6 Plus can not be updated to iOS 13.

Sorry, Dark Mode requires iOS 13 or higher. An iPhone 6 cannot be updated to iOS 13.

Also see: https://developer.apple.com/design/human-interface-guidelines/ios/visual-design/dark-mode

and

Use Dark Mode on your iPhone, iPad, or iPod touch - Apple Support