Go to the parent category and click the 
icon.
Then click the Settings tab.
Find the checkbox labeled Show subcategory list above topics in this category
If it’s not checked, check that box. Directly below that is where you set the style for it. Boxes is the style shown in your screenshot, but there are a few others as well. It’s best to try them all and see which you like best.
When you save the changes and go back to the category you may need to refresh the page to see the change as caching will sometimes make it not show the change right away.