Instructions
Turn on your console. When greeted with the main menu, use your controller's directional pad to scroll down and highlight the "Game Menu" icon.
Press your controller's "X" button to open up the Game Menu. Use your controller's directional pad to highlight any game saves you no longer need.
Scroll down to the "Delete" option on the menu that appears when a save file is highlighted. Press the "X" button on your controller. When asked whether or not you are sure about deleting the save file, highlight "Yes" and press the "X" button. Repeat this process for every save file you wish to delete.