Click on the image you want to change. Then click the delete button on your keyboard or click on the trash icon on the tool bar.
Using the image button the navigation bar on the left of your workspace you can search for new images.
Select the image type you wish to add by clicking on the icon. Scroll or use the search function to find your new image. Click on the image you want to add to your design and it will be added to your canvas.
Now you need to order your layers so that the background image is at the back and all other items are overlayed onto it. To do this click on the layer icon on the right hand side panel and click on the icon (show icon v) to set the image as the back layer.