Hard Reset HTC Desire 816

First method:

1. Turn off your phone.
2. 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 press Volume Down to select option "Factory reset", and press Power button to confirm.
5. Well done!

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. Well done!

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