Starting in Android Marshmallow, you can choose an 'assist app'. An assist app is the app that opens when you hold down your home button. Android phones usually come with the Google Assistant or Google Now. But, if you install apps like Cortana or Hound assistant, you can switch to that assistant as your default.
- Open settings
- Find Apps or Application manager
- Press the three dotted icon in the top right
- Press configure apps
- Find assist or assist & voice input
- Press assist app and choose!