- You must have a custom recovery installed on the HTC One M9 smartphone  before you can follow this guide.
- You must have updated the HTC One M9 smartphone to Android 6.0 Marshmallow before following this guide. You can check for software updates by tapping on the Menu > Settings > About > Software Updates > Check For Updates and it should be waiting if you are not running that version already.
- Likewise, you can head to the Menu > Settings > About Device just to check what version your are currently running if you have no idea.
BEFORE WE BEGIN
- You must enable the Developer Options from the Settings before starting with the guide. Do that by tapping on the Menu > Settings > About Device > Build Number 7 times and it will say that you are using the M9 now as a developer.
- You must enable the USB Debugging Mode from the Settings. You can do that now since you have the Developer Options ready. tap on the Menu > Settings > Developer Options > USB Debugging Mode.
- Rooting the HTC One M9 using any of the available methods to date – and running on any firmware — will always void the warranty you get from the HTC company. You can unroot the M9 device using a different guide and it will make the warranty work again.
HOW TO ROOT HTC ONE M9 RUNNING ON ANDROID 6.0 MARSHMALLOW
- Download the SuperSU you need for the M9 from this link  directly to the desktop of your computer.
- Connect the M9 handset to the computer using the USB cable.
- Copy the SuperSU over the root of the SD card internal storage.
- Boot your HTC One M9 up in the custom recovery mode. You can only do that one of two ways since you cannot use the Quick Boot app just yet. You can choose to enter recovery from adb commands after installing ADB on your computer, or just using the hardware keys on your device.
- Turn off the M9 device and reboot by holding the Volume Down for a few seconds and then press the Power button. Release both buttons together after a second or so and you should see the bootloader mode menu on the display.
- Highlight the recovery option from the bootloader menu and your device will enter the recovery mode of whatever recovery is on your device. In this instance it will be the TWRP Recovery if you followed our guide for installing TWRP Recovery.
- Once in the recovery mode, tap on the ‘Install’ option from the menu.
- Follow the prompts to browse the SD card for your SuperSU file.
- Go to the main recovery menu and choose the ‘Reboot System’ option to get your M9 back in the normal mode you need to use your device.
That’s all you need to root HTC One M9 on Android 6.0 Marshmallow — hopefully the same method works for future builds too. You’ll need to look out for others who are testing to see if it works. It might need another updated recovery in the future.