Periods or halves or quarters can be set when creating a game or edited for a pending game. You should not change the periods for in-progress or completed games.
First create a game or select a pending game from the list by clicking the plus to create or the small blue "i" in a circle in the right column.
Now scroll to the bottom of the settings to see the periods category.
Select the Periods in the "Data Collection" section.
Change the time or select the Periods to edit.
Note: the current version only supports 8 periods - the next release increases this to 10 periods.