You can reset the portal's monthly or overall leaderboard.
To do that:
Sign in as an Administrator.
Select the menu bars in the top-left corner.
Go to App Control (1).
On the left-side panel, choose Gamification (2).
Click on the Reset Now link next to your preferred option:
-Enable user Leaderboards (All-time): to reset the overall stats (3).
-Enable user Leaderboards (Seasonal): to reset the monthly stats (4).
Click Reset Now on the pop-up message that appears.
Note: This action cannot be reverted, so please be absolutely certain before confirming the reset process.
You can also choose to automate the reset process. See here for more.