Select the user with the gamification statistics that you wish to change. You will find a green "Gamification" button (2) on the right side.
This button will open the user's leaderboard and by clicking on the arrow next to his points (3), you are able to reset the number of his own points. If you want to reset his level and badges as well, you will need to click on the arrow next to the level and badges. Please note that this action can not be undone.