Hard Reset HTC One M8

First method:

1. Switch off your device.
2. Next press and hold Volume Down + Power button.
3. When you see the three Android images, then release held keys.
4. Then press Volume Down to select option "Factory reset", and press Power button to confirm.
5. Good job!

Second method:

1. Switch on the device.
2. Then from Main Menu go to Settings and tap Backup & reset.
3. Next choose Reset phone and select Erase all data.
4. Tap OK to confirm your choice.
5. Success!

WARNING !
Hard Reset will erase all of your data
All described operations you are doing at your own risk.