Hard Reset HTC One M8 Dual SIM

First method:

1. Switch off your device.
2. Afterwards press and hold Volume Down + Power button for a while.
3. Release held keys when you see an Android menu on your screen..
4. Then press Volume Down to select option "Factory reset", and press Power button to confirm.
5. Good job! The hard reset has just been performed.

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.