You can select a winner during any time your contest is active, or once your contest has completed. On your Entries tab, go to the Entry that you would like to select as winner. In the Actions drop down, click on the Select Winner option. You will be asked for a confirmation, and once you confirm , the winner will be immediately selected.
Note: Once a winner has been selected, it can not be changed.
Watch this video to see how to select a winner of your contest: