Click "+" button and select "Keep Only ..." menu. Then select its "Top N" or "Bottom N" submenu.
You can also select "Keep Only ..." from the column menu of the column on which you want to apply top/bottom N operation, and then select "Top N" or "Bottom N" from its submenu.
Choose between "Top" tab or "Bottom" tab.
Specify the number (N) of top/bottom rows to keep in the text field.
Select the column to apply top/bottom N operation from "Based on" dropdown.
Click "Run" button to keep only top/bottom N rows.