To change the number of periods in your planbook, click on "Planbooks" in your blue navigation bar, and then "Edit" by the planbook you're working on.  You will see a field to set the number of periods.

If you decrease the number of periods, it will take them from the bottom up.  To move your empty periods to the bottom of your planbook, click on "Planbooks" in your blue navigation bar, and then "More" by the planbook you're working on.  You will be able to reorder your periods, and then delete them following the above directions.