Hard Reset HTC One Mini

First method:

1. First of all, turn off your phone by holding the Power button.
2. Afterwards press and hold  Volume Down + Power at the same time for a few seconds.
3. Release held keys when you see the Bootloader Mode.
4. Then use Volume Down to select "Factory reset", and press Power button to confirm this option.
5. Well done! The hard reset has just been performed.

Second method:

1. Turn on the phone by using the Power button. 
2. Afterwards go to Settings and then to Backup & reset.
3. Tap Reset Phone. 
4. Choose Erase Everything if you are sure you want to clear all your data. 
5. Select OK to confirm the whole operation.
6. Good job!

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