Want to keep Notes only on my iPhone 6
If the notes are associated with your email account you'll have to copy them somewhere (you can email them, send them as text, iMessage,  copy and paste them but only one by one), delete them —which will happen automatically when you disable them on Settings > Mail, contacts, calendars— and then add them back to reside exclusively on your iPhone.
Consider making regular backups of your device, because if anything happens to it, that will be your only way of recovering them.