Once you have downloaded your images to at least two separate locations, eg., your hard drive and cloud storage you no longer need them on your SD card. The quickest and recommended way to delete them is to format your card in the camera. Where the format option is on your camera will vary from camera to camera but you will probably find it under the setup section of your menus. It will ask you for confirmation before deleting so don't be afraid to explore the menu options but if in doubt consult your operating manual or post your question here...