Hard Reset HTC One

First method:

1. Power off the device by pressing Power button.
   
2. Then hold Volume Down + Power button until you see three Android images.
   
3. Now select option "FACTORY RESET" to browse use Volume Down, and to choose optionPower button. 
4. Device will reboot itself.

Second method:

1. Go to Settings, next tap Backup & reset.
2. Select Reset phone.
3. if u want delete all data (all the media) select Erase all data.
4. Press OK.

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