Please select your existing template, you can find it in Create Design tab, it is the one that is marked.
Go to the slider section, while hovering (moving the mouse) over the image, a new feature will appear. Select replace design (marked in red):
A new window will open with your image library. You can select another image from the library or upload a new picture from your computer. To upload from your computer select the Upload button on the right:
Once you clicked on a new image, the selected image will be automatically updated.
Now you can see a preview or save this image. The Preview and Save buttons are on the right, marked in red.