When you set up your houses in the "Settings" section you can choose to end each house round manually or automatically after a certain number of days.
If you have configured your houses on an automatic schedule, you have nothing to do. CritterCoin will automatically close each round and reset your house points and issue Critter the students in the winning house.
If you house rounds are manual, you can close and start a new round any time by click the "Close Round" button.