If you have input a food or insulin action incorrectly, you have the option to delete the action through the Timeline.
To delete an action:
Press on the Timeline icon at the top right of the overview screen
Press Edit to enable deleting options
Press the - button on the action you want to delete
Press the Delete button that appears
Press Edit to hide deleting options
That's it! The action has been removed. If the food or insulin was active in your body, the overview screen will remove the item from display.