The Android Marshmallow and Sense 7 update have already hit the HTC One M8 and thanks to this update, you get to change and customize navigation buttons on HTC One M8. It means you can rearrange these touch buttons, and add or remove buttons to get it completely yours.
To customize navigation buttons on HTC One M8, do as follows:
1. Make sure your One M8 is already updated to the Android 6.x Marshmallow and runs Sense 7.0 (go to Settings > About > Software Updates to check for Marshmallow update)
2. Open the Settings app and tap the Personalize option under the Personal settings.
3. Tap the Change Navigation Buttons option, under the Navigation Bar settings.
4. Now you get to edit the navigation buttons. Press and hold right icon to rearrange and make any option to add to the navigation bar
**Note that you can not delete the Home, Recent Apps and Back buttons from the touch buttons, and the 4th spot can be assigned to either of these options: Hide navigation bar, Notifications, Quick Settings, Turn off Screen, or Auto Rotate.