Generally, we are suggested to reset our iPhone to factory settings before we sell it. You can go to "Settings > General > Reset > Erase All Content and Settings". And everything will be removed from your iPhone 6 then.
But one of my friend working in Apple told me that reset is not a safe way. In other words, it is still possible to recover. He suggested me to consider third-party data eraser to make it.
The one I used to wipe my old iPhone 5 is iMyFone iOS Eraser, you can take it for reference. It can completely erase all data from iPhone and it is unrecoverable then. Have a shot.