Hard Reset HTC Desire 620

First method:

1. First, turn off the phone by using the Power button.
2. Press and hold Volum Down and Power buttons together.
3. Wait for the three Andoid images.
4. When you see them, choose the Factory Reset by pressing the Volum Down button. 
5. Now the hard reset is done! 

Second method:

1. Turn on the phone by using the Power button. 
2. Go to Settings and then to Backup & Reset.
3. Tap Reset Phone. 
4. Choose Erase All Data if you are sure you want to clear all your data. 
5. Tap OK. 
6. Well done!

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