Hard Reset HTC One M8 Mini

First method:

1. Start by turning off the phone.
2. After that, hold down Volum Down and Power buttons together.
3. You can release held keys when you see a new menu on your screen.
4. Choose the Factory Reset by pressing the Volum Down button to navigate and Power button to select.
5. Now the hard reset is done! 

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.